Since a 70% or higher Accept Rate is considered excellent, make the Accept Rate number turn green at 70%, not 80%.
It seems, to me, that a large amount of these can relate to the comment "please improve your accept rate", or similar.
Currently, the accept rate colouring is green for 80%+, red / orange at 35% or below and grey otherwise. This implies that only an accept-rate of 80% is acceptable.
The average accept rate for users with greater than 200 reputation is 77.3%. This doesn't test for questions with an answer as the server couldn't deal with the query, which means that the actual number will be higher than this.
Removing the restriction on reputation the average accept-rate falls to 67.6%. This includes the condition that a question must have an answer.
Looking at a graph of the number of users with a certain accept-rate and ignoring the obvious spikes at 25%, 33.3%, 50%, 75%, 80% etc that are caused by users with a small number of questions there isn't really any obvious pattern.
... any accept rate of 70% or higher is considered "excellent"
and the FAQ
In general, an accept rate of 70% or better is quite good
In order to reduce the number of questions on meta about the importance of the accept-rate or comments left about it and to reduce the number of comments on the main site could an accept-rate of 70% - or 75% if people prefer - be coloured green?
As green implies good this small UI change might, on its own, be able to affect a change in behaviour.
I like 70% because it's twice 35, which pleases my brain :-).