- Take your image
- Pixelate your image. Work out the exact number of Rubiks cube cells you will have and make your image that size in pixels, so 1 cell = 1 pixel.
If you don't know what scale you want to work at, don't know how many cubes you want to use etc. Using the Mosaic effect (Filter → Pixelate → Mosiac...) will help you preview quickly.
- Set a Gradient Map adjustment layer over your image with your chosen colors. To prevent the gradient between colors just double up each color and set the next color directly next to the previous. You can see how I set up my gradient:
Your result may not work exactly as intended. You can play with the levels in your original image and the distribution of colors in your gradient map to get a better result.
It's worth noting that you will most probably still get some variations on color from the gradient map but you can manually redraw any problem areas. As a quick preview this is probably fine. If you truly want only 6 colors you can convert your image to indexed color with a palette of only the 6 colors you want.
These images can be fairly unrecognisable when you are zoomed in so if you are manually redrawing areas, use the navigator window to keep a preview visible.