The screen color is retrieved here:
vec3 screen_col = texture(TEXTURE, screen_sample_uv).rgb
After this, you could check if that color is red by some metric (ex. if red value is 4 times greater than blue/green). Then when you apply the final color step, use a red gradient texture if the screen color is red
if( <screen_col is red> ) final_col = texture(u_color_tex_reds, vec2(col_sample, 0.5)).rgb;
Something like that should work