The color of the engine depended on the date produced and also the horsepower. There were two variations. One variation was all yellow and the other was cream colored valve covers and air cleaner and a tan block. There was a Lincoln/Mercury service bulletin dated 12-17-54 #237 that described the differences. It had something to do with which heads were used on the engine and the dates.
You can get more info. from Mr. Merle Fourez at Classic Mercury Parts in California. He is the one that supplied me with the above info. His number is 805-646-3345. I had seen pictures of 55 Mercs with the cream valve covers and some with the all yellow motors and wondered what the difference was.