Generate colors between red and green for a power meter?

If you want an green-yellow-red representation like the accepted answer suggests then take a look at this.

http://jsfiddle.net/0awncw5u/2/

function percentToRGB(percent) {
    if (percent === 100) {
        percent = 99
    }
    var r, g, b;

    if (percent < 50) {
        // green to yellow
        r = Math.floor(255 * (percent / 50));
        g = 255;

    } else {
        // yellow to red
        r = 255;
        g = Math.floor(255 * ((50 - percent % 50) / 50));
    }
    b = 0;

    return "rgb(" + r + "," + g + "," + b + ")";
}


function render(i) {
    var item = "<li style='background-color:" + percentToRGB(i) + "'>" + i + "</li>";
    $("ul").append(item);
}

function repeat(fn, times) {
    for (var i = 0; i < times; i++) fn(i);
}


repeat(render, 100);

li {
    font-size:8px;
    height:10px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
<ul></ul>

Off the top of my head, here is the green-red hue transition in HSV space, translated to RGB:

blue = 0.0
if 0<=power<0.5:        #first, green stays at 100%, red raises to 100%
    green = 1.0
    red = 2 * power
if 0.5<=power<=1:       #then red stays at 100%, green decays
    red = 1.0
    green = 1.0 - 2 * (power-0.5)

The red, green, blue values in the above example are percentages, you'd probably want to multiply them by 255 to get the most used 0-255 range.

This should work - just linearly scale the red and green values. Assuming your max red/green/blue value is 255, and n is in range 0 .. 100

R = (255 * n) / 100
G = (255 * (100 - n)) / 100 
B = 0

(Amended for integer maths, tip of the hat to Ferrucio)

Another way to do would be to use a HSV colour model, and cycle the hue from 0 degrees (red) to 120 degrees (green) with whatever saturation and value suited you. This should give a more pleasing gradient.

Here's a demonstration of each technique - top gradient uses RGB, bottom uses HSV:

Short Copy'n'Paste answer...

On Java Std:

int getTrafficlightColor(double value){
    return java.awt.Color.HSBtoRGB((float)value/3f, 1f, 1f);
}

On Android:

int getTrafficlightColor(double value){
    return android.graphics.Color.HSVToColor(new float[]{(float)value*120f,1f,1f});
}

note: value is a number between 0 and 1 indicating the red-to-green condition.