[...]some seem to have a grey surround to the Apple logo on the front of the case, while others match the overall colour of the computer, is there any significance to this?

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All Mac 128ks have their Apple logo on a square piece of formed and painted aluminum that is stuck into a corresponding recess in the case. The Mac's plastic has a texture, whereas the Apple logo is smooth. It could be that a camera flash lights up the square differently than the rest of the Mac. That's all I can think of.

I understand the 128 and 512 have a 64kb ROM, was there only one version of this ROM, if there were different revisions is there any way, visual or otherwise on how to spot them?

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My records indicate that there are 2 different 128k/512k Mac ROM checksums:

0x28BA61CE

0x28BA4E50

One way to check is to use a Mac ROM dumper, such as GetROM, to save the Mac's ROM to a file. Then open it in a hex editor. The first 4 bytes of hex data is where the checksum is stored.

I'm not aware of any practical difference between these ROMs, and given that the checksums are so similar, it stands to reason that only a very small number of bytes were changed. There isn't, for example, an extra driver present, but there may be some little bug fixes here and there.