Linearly increasing color darkness algorithm

Question

Hi, I want to write a function in ruby that given a number between 1 and 500 will output a 6 digit hex color code that gets linearly darker for higher numbers. This doesn't seem that hard but I'm not sure where to begin. How can I implement this?

edit

Hue seems like a more reliable way to go. I'd like to give a reference color, say a shade of green, and then darken or lighten it based on the input number.

input: 10
output: color code (in rgb or HSV) that is a light shade of the reference color

input: 400
output: color code (in rgb or HSV) that is a fairly dark shade of the reference color

edit 2

The only reason I need to use between 1 and 500 is because that's the input I have to work with. It's alright if some numbers that are close together map to the same color.

Answer 1

Why not just return a gray level then, #ffffff to #000000? 500 levels of darkness aren't really distinguishable anyway, and grays give you 256 levels.

Answer 2

The 6 digit hex color code is in RGB. What you want is to work in HSV: pick a Hue and Saturation, and gradually decrease the Value. Convert from HSV to RGB to output the color. See here for an example.

Answer 3

If you only want to darken your reference color, it's easy. Given an R,G,B color that is the brightest you want to go, multiply each of the 3 values by (500-input) and divide by 499. Convert each of the values to 2 hex digits and append them with a # at the front.

Answer 4

Basic linear interpolation?

// Pseudocode
function fade_colour(source, factor)
    const max = 500
    const min = 1

    foreach component in source
        output[component] = round(source[component] * (max - value) / (max - min))
    endforeach

    return output
endfunction