The sns.choose_colorbrewer_palette() returns the the colorscheme you want to use. Depending on whether the as_cmap argument is true or false it will either return a list of colors ("seaborn palette") or a matplotlib colormap.

For anyone finding this on google - I ended up using the excellent spectra Python library to handle interpolation. I used this to create a helper function that is now part of my tool MultiQC. The code is on GitHub but I've pasted it below in case the link changes in the future:

#!/usr/bin/env python
"""
Helper functions to manipulate colours and colour scales
"""

from __future__ import print_function
import spectra
import numpy as np
import re

# Default logger will be replaced by caller
import logging
logger = logging.getLogger(__name__)


class mqc_colour_scale(object):
    """ Class to hold a colour scheme. """

    def __init__(self, name='GnBu', minval=0, maxval=100):
        """ Initialise class with a colour scale """

        self.colours = self.get_colours(name)

        # Sanity checks
        minval = re.sub("[^0-9\.]", "", str(minval))
        maxval = re.sub("[^0-9\.]", "", str(maxval))
        if minval == '':
            minval = 0
        if maxval == '':
            maxval = 100
        if float(minval) == float(maxval):
            self.minval = float(minval)
            self.maxval = float(minval) + 1.0
        elif minval > maxval:
            self.minval = float(maxval)
            self.maxval = float(minval)
        else:
            self.minval = float(minval)
            self.maxval = float(maxval)

    def get_colour(self, val, colformat='hex'):
        """ Given a value, return a colour within the colour scale """
        try:
            # Sanity checks
            val = re.sub("[^0-9\.]", "", str(val))
            if val == '':
                val = self.minval
            val = float(val)
            val = max(val, self.minval)
            val = min(val, self.maxval)

            domain_nums = list( np.linspace(self.minval, self.maxval, len(self.colours)) )
            my_scale = spectra.scale(self.colours).domain(domain_nums)

            # Weird, I know. I ported this from the original JavaScript for continuity
            # Seems to work better than adjusting brightness / saturation / luminosity
            rgb_converter = lambda x: max(0, min(1, 1+((x-1)*0.3)))
            thecolour = spectra.rgb( *[rgb_converter(v) for v in my_scale(val).rgb] )

            return thecolour.hexcode

        except:
            # Shouldn't crash all of MultiQC just for colours
            return ''


    def get_colours(self, name='GnBu'):
        """ Function to get a colour scale by name
        Input: Name of colour scale (suffix with -rev for reversed)
               Defaults to 'GnBu' if scale not found.
        Returns: List of hex colours
        """
        # ColorBrewer colours, taken from Chroma.js source code
        # https://github.com/gka/chroma.js

        ### Copyright (c) 2002 Cynthia Brewer, Mark Harrower, and The
        ### Pennsylvania State University.
        ###
        ### Licensed under the Apache License, Version 2.0 (the "License");
        ### you may not use this file except in compliance with the License.
        ### You may obtain a copy of the License at
        ### http://www.apache.org/licenses/LICENSE-2.0
        ###
        ### Unless required by applicable law or agreed to in writing, software distributed
        ### under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
        ### CONDITIONS OF ANY KIND, either express or implied. See the License for the
        ### specific language governing permissions and limitations under the License.

        colorbrewer_scales = {
            # sequential
            'OrRd': ['#fff7ec', '#fee8c8', '#fdd49e', '#fdbb84', '#fc8d59', '#ef6548', '#d7301f', '#b30000', '#7f0000'],
            'PuBu': ['#fff7fb', '#ece7f2', '#d0d1e6', '#a6bddb', '#74a9cf', '#3690c0', '#0570b0', '#045a8d', '#023858'],
            'BuPu': ['#f7fcfd', '#e0ecf4', '#bfd3e6', '#9ebcda', '#8c96c6', '#8c6bb1', '#88419d', '#810f7c', '#4d004b'],
            'Oranges': ['#fff5eb', '#fee6ce', '#fdd0a2', '#fdae6b', '#fd8d3c', '#f16913', '#d94801', '#a63603', '#7f2704'],
            'BuGn': ['#f7fcfd', '#e5f5f9', '#ccece6', '#99d8c9', '#66c2a4', '#41ae76', '#238b45', '#006d2c', '#00441b'],
            'YlOrBr': ['#ffffe5', '#fff7bc', '#fee391', '#fec44f', '#fe9929', '#ec7014', '#cc4c02', '#993404', '#662506'],
            'YlGn': ['#ffffe5', '#f7fcb9', '#d9f0a3', '#addd8e', '#78c679', '#41ab5d', '#238443', '#006837', '#004529'],
            'Reds': ['#fff5f0', '#fee0d2', '#fcbba1', '#fc9272', '#fb6a4a', '#ef3b2c', '#cb181d', '#a50f15', '#67000d'],
            'RdPu': ['#fff7f3', '#fde0dd', '#fcc5c0', '#fa9fb5', '#f768a1', '#dd3497', '#ae017e', '#7a0177', '#49006a'],
            'Greens': ['#f7fcf5', '#e5f5e0', '#c7e9c0', '#a1d99b', '#74c476', '#41ab5d', '#238b45', '#006d2c', '#00441b'],
            'YlGnBu': ['#ffffd9', '#edf8b1', '#c7e9b4', '#7fcdbb', '#41b6c4', '#1d91c0', '#225ea8', '#253494', '#081d58'],
            'Purples': ['#fcfbfd', '#efedf5', '#dadaeb', '#bcbddc', '#9e9ac8', '#807dba', '#6a51a3', '#54278f', '#3f007d'],
            'GnBu': ['#f7fcf0', '#e0f3db', '#ccebc5', '#a8ddb5', '#7bccc4', '#4eb3d3', '#2b8cbe', '#0868ac', '#084081'],
            'Greys': ['#ffffff', '#f0f0f0', '#d9d9d9', '#bdbdbd', '#969696', '#737373', '#525252', '#252525', '#000000'],
            'YlOrRd': ['#ffffcc', '#ffeda0', '#fed976', '#feb24c', '#fd8d3c', '#fc4e2a', '#e31a1c', '#bd0026', '#800026'],
            'PuRd': ['#f7f4f9', '#e7e1ef', '#d4b9da', '#c994c7', '#df65b0', '#e7298a', '#ce1256', '#980043', '#67001f'],
            'Blues': ['#f7fbff', '#deebf7', '#c6dbef', '#9ecae1', '#6baed6', '#4292c6', '#2171b5', '#08519c', '#08306b'],
            'PuBuGn': ['#fff7fb', '#ece2f0', '#d0d1e6', '#a6bddb', '#67a9cf', '#3690c0', '#02818a', '#016c59', '#014636'],

            # diverging
            'Spectral': ['#9e0142', '#d53e4f', '#f46d43', '#fdae61', '#fee08b', '#ffffbf', '#e6f598', '#abdda4', '#66c2a5', '#3288bd', '#5e4fa2'],
            'RdYlGn': ['#a50026', '#d73027', '#f46d43', '#fdae61', '#fee08b', '#ffffbf', '#d9ef8b', '#a6d96a', '#66bd63', '#1a9850', '#006837'],
            'RdBu': ['#67001f', '#b2182b', '#d6604d', '#f4a582', '#fddbc7', '#f7f7f7', '#d1e5f0', '#92c5de', '#4393c3', '#2166ac', '#053061'],
            'PiYG': ['#8e0152', '#c51b7d', '#de77ae', '#f1b6da', '#fde0ef', '#f7f7f7', '#e6f5d0', '#b8e186', '#7fbc41', '#4d9221', '#276419'],
            'PRGn': ['#40004b', '#762a83', '#9970ab', '#c2a5cf', '#e7d4e8', '#f7f7f7', '#d9f0d3', '#a6dba0', '#5aae61', '#1b7837', '#00441b'],
            'RdYlBu': ['#a50026', '#d73027', '#f46d43', '#fdae61', '#fee090', '#ffffbf', '#e0f3f8', '#abd9e9', '#74add1', '#4575b4', '#313695'],
            'BrBG': ['#543005', '#8c510a', '#bf812d', '#dfc27d', '#f6e8c3', '#f5f5f5', '#c7eae5', '#80cdc1', '#35978f', '#01665e', '#003c30'],
            'RdGy': ['#67001f', '#b2182b', '#d6604d', '#f4a582', '#fddbc7', '#ffffff', '#e0e0e0', '#bababa', '#878787', '#4d4d4d', '#1a1a1a'],
            'PuOr': ['#7f3b08', '#b35806', '#e08214', '#fdb863', '#fee0b6', '#f7f7f7', '#d8daeb', '#b2abd2', '#8073ac', '#542788', '#2d004b'],

            # qualitative
            'Set2': ['#66c2a5', '#fc8d62', '#8da0cb', '#e78ac3', '#a6d854', '#ffd92f', '#e5c494', '#b3b3b3'],
            'Accent': ['#7fc97f', '#beaed4', '#fdc086', '#ffff99', '#386cb0', '#f0027f', '#bf5b17', '#666666'],
            'Set1': ['#e41a1c', '#377eb8', '#4daf4a', '#984ea3', '#ff7f00', '#ffff33', '#a65628', '#f781bf', '#999999'],
            'Set3': ['#8dd3c7', '#ffffb3', '#bebada', '#fb8072', '#80b1d3', '#fdb462', '#b3de69', '#fccde5', '#d9d9d9', '#bc80bd', '#ccebc5', '#ffed6f'],
            'Dark2': ['#1b9e77', '#d95f02', '#7570b3', '#e7298a', '#66a61e', '#e6ab02', '#a6761d', '#666666'],
            'Paired': ['#a6cee3', '#1f78b4', '#b2df8a', '#33a02c', '#fb9a99', '#e31a1c', '#fdbf6f', '#ff7f00', '#cab2d6', '#6a3d9a', '#ffff99', '#b15928'],
            'Pastel2': ['#b3e2cd', '#fdcdac', '#cbd5e8', '#f4cae4', '#e6f5c9', '#fff2ae', '#f1e2cc', '#cccccc'],
            'Pastel1': ['#fbb4ae', '#b3cde3', '#ccebc5', '#decbe4', '#fed9a6', '#ffffcc', '#e5d8bd', '#fddaec', '#f2f2f2'],
        }

        # Detect reverse colour scales
        reverse = False
        if str(name).endswith('-rev'):
            reverse = True
            name = name[:-4]

        # Default colour scale
        if name not in colorbrewer_scales:
            name = 'GnBu'

        # Return colours
        if reverse:
            return list(reversed(colorbrewer_scales[name]))
        else:
            return colorbrewer_scales[name]

This function can then be used as follows:

c_scale = mqc_colour.mqc_colour_scale(scale_name, min_val, max_val)
hex_code = c_scale.get_colour(val)

Note that the code in its current state returns a very washed out version of the colour, for use as a background colour behind black text. You should be able to easily remove this from the get_colour function if desired though.

I hope this helps someone!

Phil