[papalalu]

It's not just something like changing colours, though i have had to do that - it's applying the colours in the first place.

Rather than trying to remember a previous hex, or looking it up, you can think ok, this should be the darker highlight colour - and as you say, looking later you know which colour that is.

It's not just colours - a good example is a clearfix. Rather than add it to the markup, you can add it via css, and of course if you change it, you change it once.