If you select a batch of images of the same target that are clearly taken with different colour filters, you start to see some patterns.

the number after the L or R is the filter number - typically we get 2, 5, & 6 for RGB, but I don't know what order they are. Actually, I tell a lie. From the response of the blue tab on the colour chart it is clear that "2" = blue, and it then looks like "5"=green, and "6"=red

Ah, here's what you need:- http://athena.cornell.edu/pdf/tb_pancam.pdf

# Left Right
1 - 430 nm
2 750 nm 750 nm
3 670 nm 800 nm
4 600 nm 860 nm
5 530 nm 900 nm
6 480 nm 930 nm
7 <430 nm >980 nm
8 440 S 880 S

S= dark filter for Sun viewing (that's how they get pictures without frying the CCD)

You'll also see that some of the digits change between consecutive images - these are probably the "frame numbers" of the difital data sent from mars. There will also be a code for Spirit.