This fish can alter its patterning and colouration for several reasons. I would check the water first, just to be certain. This species is highly intolerant of nitrates and any substantive changes in quality or parameters. Provided ammonia and nitrite are zero, and nitrate is no higher than 10ppm max, and the pH has not shifted from the norm for your tank, the cause is then most likely either from interaction with the other ram or sometimes other fish species.
I have a male Bolivian who is now just approaching five years, so he is at or slightly past the normal lifespan, and over the five years he has changed like this now and again.
If the yellow is intensifying, this could be pre-spawning. Do you know if you have a pair?
Byron Hosking, BMus, MA
Vancouver, BC, Canada
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]