Conditional formatting -- percentage to color conversion

What you probably want to do is to assign your 0% to 100% some points in a HSV or HSL color-space. From there you can interpolate colors (and yellow just happens to be between red and green :) and convert them to RGB. That will give you a nice looking gradient between the two.

Assuming that you will use the color as a status indicator and from a user-interface perspective, however, that is probably not such a good idea, since we're quite bad at seeing small changes in color. So dividing the value into, for example, three to seven buckets would give you more noticeable differences when things change, at the cost of some precision (which you most likely would not be able to appreciate anyway).

So, all the math aside, in the end I'd recommend a lookup table with the following colors with v being the input value:

#e7241d for v <= 12%
#ef832c for v > 12% and v <= 36%
#fffd46 for v > 36% and v <= 60%
#9cfa40 for v > 60% and v <= 84%
#60f83d for v > 84%

These have been very naïvely converted from HSL values (0.0, 1.0, 1.0), (30.0, 1.0, 1.0), (60.0, 1.0, 1.0), (90.0, 1.0, 1.0), (120.0, 1.0, 1.0), and you might want to adjust the colors somewhat to suit your purposes (some don't like that red and green aren't 'pure').

Please see:

• Using HSL Color (Hue, Saturation, Luminosity) To Create Better-Looking GUIs for some discussion and
• RGB and HSL Colour Space Conversions for sample C# source-code.

I made this function in JavaScript. It returns the color is a css string. It takes the percentage as a variable, with a range from 0 to 100. The algorithm could be made in any language:

function setColor(p){
    var red = p<50 ? 255 : Math.round(256 - (p-50)*5.12);
    var green = p>50 ? 255 : Math.round((p)*5.12);
    return "rgb(" + red + "," + green + ",0)";
}