There are a lot of color pickers/generators out there. Most I list here allow you to enter in your own color like you specified:

• ColorKitty - Palette from image
• Cohesive Colors - New palette formed by old palette + color overlay
• Colour Lovers - Palettes based on color or combination
• Handy Colors - Palettes from named colors (handy for testing) and some pre-made palettes
• Picular - colors from search terms
• ColorBox- Generator by Lyft
• Data Color Picker - Generator ("specifically for data visualization")
• Colormind - Generator that uses UI to show the colors
• Couleur - Generator
• Palx - Generator ("specifically for UI")
• Geenes - Generator
• Color by HailPixel - Generator
• ColourCo.de - Generator
• Adobe Color CC - Generator
• Coolors - Generator
• Palette App - Generator
• Color Supply - Generator of 2-color palettes
• Poolors - Color combos least used by designers.

There are also some sites that give you pre-made palettes, including those of popular brands

• Google's Material palette generator - Generate material color templates according to their design guidelines ^{(well, close)}
• Brand Colors - Brand colors
• Happy Hues - Palettes that you can preview applied to a webpage
• Colors.lol - Overly descriptive color palettes.
• Culrs - Picked palettes based on color type
• Poke Palettes - Palettes based on Pokemon
• Color Leap - Palettes based on time periods
• Found Color - Palettes from curated photos
• The Day's Color - New palette daily
• Minimalist Color Palettes - A great selection of hand picked palettes
• Website color schemes - Color schemes with adjective descriptions
• Colors - computer generated palettes
• Color Hunt - Mutliple palettes daily
• Conic.css - Conic gradient pre-made palettes.

There are also some sites that offer gradient combinations which you can get the color codes from:

• CSSGradient - gradient generator
• uiGradients - probably my favorite gradient site
• HD Gradients
• Hypercolor
• Grabient
• Webgradients
• Eggradients
• Color Designer
• Gradient Magic
• Colors - AI palette generator based on text prompts

Or you could let AI choose some colors for you using AI Color Wheel.

I also like to look at the chart below (pulled from here when considering which colors to mix (usually on top of the gradients above) for best contrast:

P.S. Similar but different, some tools to interpolate between two colors:

• Non-boring gradients
• tinygradient