I am a replacing rheem 6 wire thermostat (model unknown) with rth3100c thermostat. Where do wire go? Colors are blue, orange, red, white, green and yellow.

Write down where the wires went on the old thermostat then follow the manufactures instructions that came with your new thermostat.

Just in case you lost your manual

http://customer.honeywell.com/techlit/pdf/PackedLit/69-1889EF.pdf