I am installing a new Honey well rth110b Thermostat replacing and old mercury switch and the colors do not match.
The existing wire colors are Red, Yellow, Orange, Black and Brown.
The new Thermostat is labeled ,
Rh, Rc, W, Y, G!

Did you not write down what wire went where on the old stat? If not the only way to tell is go to the furnace and see what they are hooked too.

Red to defrost control, black to impedance relay ,orange to defrost relay, Brown to impedance Relay and yellow to contactor at L1 and L2.

Is this a heatpump package unit? What is brand name, and model number?