Ok, I am building as close to possible as I can 68 Nickey clone. It is time to have my motor done and I am going either way on yellow or orange block and heads. There is an old post I found on the Super Care Registry that talks about yellow Nickey blocks. No one ever gives a deffinate answer. They say they painted yellow instead of orange so you could see leaks easier. Here is the post I found. You have to read way down before it is discussed:
http://www.yenko.net/ubbthreads/showflat.php?Cat=0&Board=nickey&Number=2643&page=6&fpart=all

You may have to copy paiste it to our browser.

i went to Nickey many times in the old days and bought parts there. I never saw a yellow block.

Any one else seen the yellow block?

There is a 1967 article on a 67 427 Nickey Camaro and they are installing a 427 engine painted yellow but not all 427 Nickey built cars had yellow blocks. It may have been done as a customer request or on a car that Nickey wanted to highlight the engine swap.

I have the 1968 Nickey Lane 1:18 street and drag models. The street has the orange and the drag has the yellow block. These models are supposed to be pretty accurate although they have 67 consoles in them as well.