Zee,
After the water heater lights, if you can see the burner flame, it should be a crisp, blue flame with an inner blue cone
configuration. There could be traces of orange in the tips of the flame, but a yellow flame causes sooting. This usually means that there is a good probability of a clogged orifice, bad propane pressure or valve, bad air/fuel mixture, or a spider in the burner assembly.
Please be very careful while investigating this (and any LP appliance). For safety reasons, you may want to leave this to a trained technician.