If you are using any color [] to processing the images on screen or if you are processing the pixel [] you can work it out pixel by pixel


bitshifting is way faster than red(), green() or blue()
loadPixels();
for (int i=0;i<pixels.length;i++){
color c = pixels[i];
  float rr = c >> 16 & 0xff;  //red
float  gg = c >> 8 & 0xff;  //blue
float    bb = c & 0xff;       //green
color invertcolor = color (255-rr,255-gg,255-bb);
pixels[i]=invertcolor;
}
updatePixels();

This is only effective if you are manipulating pixels in a given scene.

phi.lho how do you know that stuff?

for those who are interested, here are some nice examples about the secret of bit-operations :)

http://www.java2s.com/Tutorial/Java/0060__Operators/0300__Bitwise-Operators.htm

It will be faster because color operations in Processing respect the colorMode, so do extra math, even in the default mode.
The resource usage should be similar.