Yep, that's the biggest issue with building older sets IMO (and being colourblind). I've messed up many Technic builds by confusing the greys and black.The VW Combi van is the only set I have done with no numbers. What made it worse is the instruction manual is printed with off colours so a light grey is actually dark, dark grey is black etc. Was a nightmare to sort through, luckily the build was awesome.
Some of the newer "adults only" (LOL) sets apparently have instructions on a black background (like Architecture sets) which make it harder to discern colours. Not sure why they've done that, as surely they use more ink?