How to code a Colorlize filter by shader?

No.
I already know that.....
But I don't want to get the brightness value of the pixel A.
I want to have another pixel B that have the same brightness as A.

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I have seen the codes of Ycocg from nvidia's site.
The code is simple, but no artist tools for convert dds file from rgb to ycocg.

 

Thanks a lot.

First try to palette textures using texture lookup
I think it's easier than color space tricks.

//======================================
uniform sampler2D idx; // image in gray color as index
uniform sampler2D pal; // 256X4 Map as palletize map.
// It's a grad map , white to black,
// then colorize it in photoshop
varying vec2 UV;
void main(void)
{
float index = texture2D(idx, UV).y; //get index , a float between 0.0~1.0
vec2 newUV = vec2(1.0-index , 0.0); //use index as UV.x
newUV.x = clamp(newUV.x , 0.1 , 0.9);
vec4 color = texture2D(pal,newUV); //get palletize color
gl_FragColor = vec4(color.xyz, index.w); //done
}
//======================================