Try using something like:
#e7eccc (R: 231, G: 236, B: 204) instead of white,
#8c1f00 (R: 140, G: 31, B: 0) instead of red,
#0c183e (R: 12, G: 24, B: 62) instead of blue,
#d29b00 (R: 210, G: 155, B: 0) instead of yellow,
#11120c (R: 17, G: 18, B: 12) instead of black
If it still doesn't feel right, try randomly filling the rectangles with dots slightly darker then the color, creating some noise, or apply a pattern, previously created in Photoshop or even Paint instead of the color fill.
I don't know the possibilities of the bitmap API, but if you can use layers, then I would apply some transparent layers with some semi-transparent grey slightly artistic brush strokes, but it involves using Photoshop.