We only have wire colors on the old thermostat and can not match up the letters with the new thermostat (Honeywell RTH6300B)

NOTE your old thermostat is a line voltage unit see below.

http://customer.honeywell.com/techlit/pdf/PackedLit/60-2241.pdf

Your new thermostat is for 24 volt low voltage. IT IS NOT GOING TO WORK IN YOUR APPLICATION. Read below.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1717.pdf