Plotly does not appear to have such a method, so I wrote one:

import plotly.colors

def get_continuous_color(colorscale, intermed):
    """
    Plotly continuous colorscales assign colors to the range [0, 1]. This function computes the intermediate
    color for any value in that range.

    Plotly doesn't make the colorscales directly accessible in a common format.
    Some are ready to use:
    
        colorscale = plotly.colors.PLOTLY_SCALES["Greens"]

    Others are just swatches that need to be constructed into a colorscale:

        viridis_colors, scale = plotly.colors.convert_colors_to_same_type(plotly.colors.sequential.Viridis)
        colorscale = plotly.colors.make_colorscale(viridis_colors, scale=scale)

    :param colorscale: A plotly continuous colorscale defined with RGB string colors.
    :param intermed: value in the range [0, 1]
    :return: color in rgb string format
    :rtype: str
    """
    if len(colorscale) < 1:
        raise ValueError("colorscale must have at least one color")

    if intermed <= 0 or len(colorscale) == 1:
        return colorscale[0][1]
    if intermed >= 1:
        return colorscale[-1][1]

    for cutoff, color in colorscale:
        if intermed > cutoff:
            low_cutoff, low_color = cutoff, color
        else:
            high_cutoff, high_color = cutoff, color
            break

    # noinspection PyUnboundLocalVariable
    return plotly.colors.find_intermediate_color(
        lowcolor=low_color, highcolor=high_color,
        intermed=((intermed - low_cutoff) / (high_cutoff - low_cutoff)),
        colortype="rgb")

The challenge is that the built-in Plotly colorscales are not consistently exposed. Some are defined as a colorscale already, others as just a list of color swatches that must be converted to a color scale first.

The Viridis colorscale is defined with hex values, which the Plotly color manipulation methods don't like, so it's easiest to construct it from swatches like this:

viridis_colors, _ = plotly.colors.convert_colors_to_same_type(plotly.colors.sequential.Viridis)
colorscale = plotly.colors.make_colorscale(viridis_colors)

get_continuous_color(colorscale, intermed=0.25)
# rgb(58.75, 80.75, 138.25)