Changing colors in a shader - opengl

Question

I have a Grey Scale Shader for the Shaders Mod for Minecraft. I have the final.fsh, and it works real nice with this resource pack:
"Craftboy Grey"
However, I'd like to modify the shader to show up in green, similar to the grey does.
"Craftboy Green"
The reason I need the shader, is to modify all the colors the resource pack cannot, and also change other player's skins to the same scale, without them needing to manually do it.
Here's the code for the shader:
// Grayscale shader by daxnitro.
// Small edit by Edrem
// It makes the green brighter =D
uniform sampler2D sampler0;
uniform sampler2D sampler1;
uniform sampler2D sampler2;
uniform float near;
uniform float far;
float getBrightness(vec4 color);
float getDepth(vec2 coord);
void applyEffect() {
float brightness = getBrightness(gl_FragColor);
gl_FragColor = vec4(brightness, brightness, brightness, gl_FragColor[3]);
}
void main() {
vec4 baseColor = texture2D(sampler0, gl_TexCoord[0].st);
gl_FragColor = baseColor;
float depth = getDepth(gl_TexCoord[0].st);
if (gl_FragColor[3] == 0.0) {
gl_FragColor = gl_Fog.color;
}
applyEffect();
}
float getBrightness(vec4 color) {
return color[0] * 0.299f + color[1] * 0.587f + color[2] * 0.114f;
}
float getDepth(vec2 coord) {
float depth = texture2D(sampler1, coord).x;
float depth2 = texture2D(sampler2, coord).x;
if (depth2 < 1.0) {
depth = depth2;
}
depth = 2.0 * near * far / (far + near - (2.0 * depth - 1.0) * (far - near));
return depth;
}

Answer 1

To make it "green scale" instead of grey scale, write the brightness only to the green component of the output:
void applyEffect() {
float brightness = getBrightness(gl_FragColor);
gl_FragColor = vec4(0.0, brightness, 0.0, gl_FragColor[3]);
}
If you want more overall brightness while still having the whole thing tinted green, you can add some brightness back in the red and blue components. For example:
gl_FragColor = vec4(brightness * vec3(0.5, 1.0, 0.5), gl_FragColor[3]);