actually you can change the colors and the good news is that now the colors won't reset if you delete a folder.
the code is below, replace
Jokes with the true name of the folder;
replace
123x56v8 with the code your account shows in the status bar when you hover over a folder link and (obviously) change the colors as you see fit.
for a top level folder:
Code:
a[href^="/mail/Jokes/?u=123x56v8"] {
background-color:hotpink;
/*
THIS LINE IS OPTIONAL: color:white!important;
*/
}
for a nested folder:
Code:
a[href^="/mail/Storage.Jokes/?u=123x56v8"] {
background-color:hotpink;
/*
THIS LINE IS OPTIONAL: color:white!important;
*/
}