I'm wondering if putting Sherwin Williams Ultra 7000 CC639 would be a bad idea over PPG Deltron DBU basecoat.

Bad idea to change brands part way thru a paint job, can lead major problems. Rule of thumb is same manuf. from primer to topcoat. That being said, if you must, spray a test piece, let it flash and watch for any reaction.

There will be no "reaction" it just isn't the best way to go. Really? they are sooooo close in technologies it isn't going to blow up in your face or anything like that.

I am the first to say, over and over, stick with the same system ALL the way thru.

The questions I have to ask are, how big is the project, how important the project is, and why the heck is this being thought of?

It isn't out of the question understand, but those are the questions I would be asking myself.

Had purchased SW for another paint job and had clear left over. Wanted to use SW for painting a couple of doors but they didn't have a paint to match so I had to go with PPG basecoat.

I've mixed brands in the past without problems, but when you do that any warrantee is void. Many companies now offer universal clears for any manufacturer's basecoat and back the product with a lifetime guarantee, so it is indeed possible to use a different clear than recomended by your basecoat manufacturer. Failures do occur though and peeling clearcoat isn't pretty. When you mix components you're on your own. Maybe someone here has used SW clear over ppg and can share the results, that's the only way you'll know if it will work. Years ago I had a custom painter over doing some sublet work for me here in my shop, the guy is unbelievably talented with an airbrush and pinstripe. To my surprise his selection of materials was manufactured by every paint company you could imagine. He applied this over than, mixed bases from one manufactuere with another, made his own transparent colors from HOK mixed with another's clear, etc. His work is awesome and holds up just fine. Just shocking to see him use all the different stuff together, been doing it forever. No problems.

My last shop about 3 years ago was in the process of switching from S-W to PPG. Did this mainly cause lack of tech support from S-W.

A period of 2-3 months as we were getting switched and finding out products we liked, we sprayed many PPG S-W combos. Never once had a reaction!!!

Also during the time that the shop used S-W every once in awhile it would seem that we just couldnt get a color match with S-W base. Order some throu PPG and get a match.
S-W undercoats, PPG basecoat then S-W clear CC630, CC635.
My Mazda truck is painted this way, kinda a sea foam green, tried forever to get a match with S-W. Bought some PPG base then celared with S-W, being about 4-5 yrs no problems.

One thing I must ask Why DBU and NOT DBC???
DBC is more friendlier to work with and in my opinion looks better too, DBU to me looks dirty compared to DBC...Eric