You can find all the different source images in the high rise catalog. I ended up grabbing the newest wallpapers (which are in RGB format) simply because they are the prettiest.

Many of the most attractive surface images are purposely false color shots so that certain features can be easily observed.

Also, correct me if I am wrong, but most color pictures from space missions (at least as far as I know) are made by superimposing monochromatic images taken with red, green and blue filters, and then combined. Before superimposing, the red, green and blue channels are in grayscale, and thus the confusion that the images are colorized to whatever NASA engineers want, but in fact, the color/merged image shows the real coloration. The reason images are taken in grayscale is because monochromatic CCDs have better resolution than color, Also transmitting grayscale images is less resource intensive for the probes than color pictures.