Hm. Well the 2, actually 3 options I can think of are:
1-lay out a thin layer of putty, use a pin or hobby knife to carve out the mosaic bits/grout lines. Not so much carve as create a slight indent.
2-cut out a bunch of little square and triangle pieces out of cardboard/plasticard/hardened putty and glue those down making sure the edges follow the design.
I think the putty would be the easier method but the 2nd method would yield better results for most of the non elite sculpters like me.
3-Also there\'s many good mosaic floors in many buildings, perhaps even seperate plastic sheets, you may want to look into purchasing or taking a greenstuff mould of a friends terrain piece or something.
Then to paint them up, regardless of how you get the mosaics done, start with the colour of the grout, cover everything, then paint the individual tiles their colours. Might be a good idea to look up some pictures of the real world mosaics you want to simulate.
I love that preacher fig, g\'luck with it, hope you post some pics when you\'re done.