Generate colors between red and green for a power meter?

I'm writing a Java game and I want to implement a power meter for how hard you are going to shoot something.

I need to write a function that takes a int between 0 - 100, and based on how high that number is, it will return a color between Green (0 on the power scale) and Red (100 on the power scale).

Similar to how volume controls work:

What operation do I need to do on the Red, Green, and Blue components of a color to generate the colors between Green and Red?

So, I could run say, getColor(80) and it will return an orangish color (its values in R, G, B) or getColor(10) which will return a more Green/Yellow RGB value.

I know I need to increase components of the R, G, B values for a new color, but I don't know specifically what goes up or down as the colors shift from Green-Red.

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Progress:

I ended up using HSV/HSB color space because I liked the gradiant better (no dark browns in the middle).

The function I used was:

public Color getColor(double power)
{
    double H = power * 0.4; // Hue (note 0.4 = Green, see huge chart below)
    double S = 0.9; // Saturation
    double B = 0.9; // Brightness

    return Color.getHSBColor((float)H, (float)S, (float)B);
}

Where "power" is a number between 0.0 and 1.0. 0.0 will return a bright red, 1.0 will return a bright green.

Java Hue Chart:

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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:

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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.

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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.

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