Trying to convert Integer range to RGB color

Question

I know you guys are all very busy, so I'm gonna keep it short and to the point.

I'm currently developing a little game for fun. There's enemys in the game. For simplicity, think about them as colored squares. I want to minimize any HUD, so I decided to display the hit points of the creatures smoothly via their color (green for healthy, yellow for damaged, red for nearly dead).

See image :

I however really struggle to come up with a decently efficient method that converts an (int) hp-value to an RGB-color. Mapping to a single int from 0-255 would not be a problem at all - here's an example function that does exactly that:

public int mapHpToGreyscale(int input) {

    //input = current hp
    double minHealth = 0;
    double maxHealth = hpmax;
    double minColValue = 0;
    double maxColValue = 255;

    int output = (int) ((input - minHealth) / (maxHealth - minHealth) * (maxColValue - minColValue) + minColValue);

    return output;
}

Is there a quick and easy way to achieve what I want to do? I'd appreciate any input.

Answer 1

import java.lang.Math;

public class Color {
    public int r;
    public int g;
    public int b;
    public Color() {
        r = 0;
        g = 0;
        b = 0;
    }
};

public static Color hpToColor(float hp, float maxhp) {
    Color color = new Color();
    float alpha = hp / maxhp;

    if (alpha <= 0.5) {
        color.r = 255;
        color.g = Math.round((alpha * 2) * 255);
        color.b = 0;
    } else {
        color.r = Math.round(((1 - alpha) * 2) * 255);
        color.g = 255;
        color.b = 0;
    }

    return color;
}