REQ: wiring diagram of volvo 850 '97 rear lights , and colour codes.

Get yourself a Haynes manual for your car. If it's like the one for my 1999 V70, it will have a fairly comprehensive set of wiring diagrams.

Assuming the 850 has a bulb failure unit like the later V70, you'll have to tap into the *input* side of this for those items which it monitors - notably tail and stop lamps. The rest can be connected very close to the lighting clusters - where it's fairly easy to work out what's what.