What I remember was that they were a light blue on the air filter housing and the rocker covers.Ford used engine color to ID engines for quite a while. The color can change with different applications too. It is hard to find engine color information after the point that they started offering multiple types of engines. The 312 was a basic Mercury Monterey type engine in 1959. Most of the fancier design Mercury cars were being offered with the Lincoln Y-blocks and the big MELs except that 1958 was the last year they used the Lincoln 368 engine.