250's didn't get painted green from the factory. Some of the early 292's were shot with Detroit Diesel green, also known as Allison Green. You can still buy that color at NAPA.
The easiest way to tell between a 292 and the other size motors is to measure the tappet side covers. The 292's is about 5 inches tall (wider than your hand is) and the others, 194, 230, 250's are about 3.5 to 4 inches tall. Or you can look at the distance between the top of the water pump and the bottom of the head. A 292 has about 1.5 inches difference, while the others are something over 1/4 inch.
The 292 my '63 came with has recieved a green paint job every time I've refreshed the motor. I get way too many comments on how ugly it is to paint it any other! The factory 250's were all Chevy orange until they came out with that corporate blue (now it's my turn to say yuk!)
I do agree about the possibility of a rebuilder shooting the engine with whatever they had or whatever the customer thought was cool.
