Have heard from reliable sources that the color did exist from the factory. Anybody seen one? More importantly (not that we don't believe you) anybody have a picture?
Several Chevy truck books site Robert Hensel, Technical Advisor Coordinator for the Vintage Chevrolet Club of America when listing the 1955-56 Jobmaster 261 engine color as yellow. The 1957 261 is also listed as a carryover yellow engine. It also came in green which is all I have seen pictures of. Blue, orange and greys ones I have seen all have been repainted so not original.