I never asked Bill Thomas that specific question, but did ask Don Swiatek.
While there may be some validity to the opinions posted, it was mainly done in order to have their engines stand out. A customer option for conversions was offered where NicKey would paint the engine any color you wanted. Since there were several magazine articles, including a cover shot with the a "Yellow" engine, Yellow became a popular color if and when the traditional Chevy Orange was not used.
The Yellow painted engines became associated with NicKey and Bill Thomas conversions.
