Recolouring SDL2 textures - c++

Question

So I have a sprite image and a mask sprite:
(the mask is actually a paletted image, where each value is a different recolour area, 0 being the blue that isn't recoloured)
These areas would be recoloured into any arbitrary colour.
What's the best way to implement this, assuming the sprite image is being cached/preloaded as an SDL_Texture? I've been given some hints, e.g. rendering to texture, alpha blending, but none seem to fit with all the "requirements"

Answer 1

I can think in two ways to achieve this:
Easier way
You can separate each recolour area from the mask in multiple textures, in such way each mask texture having only two colors, one to be the transparent (alpha=0) and the other being white (255,255,255,255). Then for each time you need to recolour you set the color mod for each mask to the desired ones and the combine with the source texture, like this:
SDL_SetRenderTarget(r, destiny);
SDL_SetBlendMod(r, SDL_BLENDMODE_NONE);
SDL_RenderCopy(r, source, destiny);
SDL_SetBlendMod(r, SDL_BLENDMODE_BLEND);
for(/* Each mask as m*/){ */
SDL_SetColorMod(m, c[i].r, c[i].g, c[i].b);
SDL_RenderCopy(r, source, destiny);
}
Alternative
Other way is to do that combination by hand, which would avoid the previous separation, mapping the each color area to the final color in some array or hash map and going through the pixel of the mask and the image and recolouring if you need to. This way in easier to implement if you use surfaces and later convert to texture, but you can do both ways, so I will explain it in pseudo-code:
s: source;
d: destiny;
m: mask;
c: [0x00ff00: 0xff000000, 0x0000ff: 0xdeadbeef];
for(i=0; i< s.pixels.size(); ++i){
if(c.count(m.pixels[i])){
destiny = (c[m.pixels[i]]+s.pixels[i])/2;
} else {
destiny = s.pixels[i];
}
}
In both examples I used the common blend function to combine, but you can try others, I think add could be a good alternative too.