I sprayed diamont for the last 4-5 years before changing to Onyx last month. It is an awesome paint line. Colors are very good, look at standard and the MPV (most popular variants) for the most part and you'll have no problems. Another handy tool is the recall formula's in smarttrack, you can see what the previous painter used and if he went with variants on a particular color code. If you have the spray out deck (colormax 2) use it, they are sprayed out cards and give you a good idea of the actual color you'll get.
I found diamont to work best when applied fairly wet, it should be covering in 2-3 coats for the most part. I always sprayed my
metallics using an orientation coat of BC100 or over-reduced clear to help make blending easier. I found I was tinting maybe one in 100-200 formula's, theres almost always a good variant if standard isn't working for you. Lastly if you aren't crazy about the
RM clears look into the
Glasurit line, they are priced very closely but perform much better IMO.