Just out of curiosity. why not use fluor yellow rims? Brands sometimes hues slightly different colours/hues, but other than that it just seems as if dirt can't stop the yellow from being bright
PS: I must admit, I use white rims too. However, once my current wheels are just too far gone I will switch to yellow for its good colour retention.
PS2: I can second what's been said about rough surfaces attracting dirt: I race Tamiya rims and they have a texture on them out of the package. You can get them relatively clean again, but they get stained very easily (you can't just wipe off the dirt like you can with smooth surface wheels).