Irrlicht Engine

Hi, i 'm learning how do post processing.
I got sucess with blur, but i can't do a simple invert color with sucess... :(

That's the fragment shader (i omit the variable declarations) ...

void main(void)
{
vec4 color = texture2D(RT, vTexCoord.xy );

gl_FragColor= vec4(1-color.r,1-color.g, 1 ...