The green Playstations are also testing stations and can run burned discs from any region, but as they were meant for testing the finished game they lack the extra memory. Functionally they should be identical to a chipped grey retail unit (and in fact, many companies used chipped Playstations due to them being a lot cheaper).
The black machines are Yaroze machines, they lack region protection but are otherwise identical to retail hardware. The full package comes with a PC link cable, a special memorycard and the Yaroze software.
White development machines contain MPEG-decoder chips (like the external VCD carts).
Were there more types of machines?