Thanks NoSparks, but I am not sure that is correct.
C8 in HEX is 200 and FF in Hex is 255,
So it is the C8FFFF (i.e. the last 6 digits before the final &) that represents the colour of RGB (255, 255, 200) - the order is reversed for the 3 colours.
Its the first digits &H00 that are puzzling and why the same colour in Excel has a code of &H80000018&

Your link was useful though as it says that Excel only recognizes names for Color 1 through 8 (Black, White, Red, Green, Blue, Yellow, Magenta, and Cyan).

This probably explains it as Excel is clearly using a completely different system for storing the colour codes as is used by Access.