I charge for labor&various personal supplies used w/the benefit being the customer pays for the core supplies. So far, in 18yrs I've been able to list which/what brand to get&where. Or how many times it's saved me from taking the hit upon blowing up into so much more as it inevitably does every so often. Saved me from myself as well&seen more than once; Either doing bit more as favor/simply being nice OR have you believe/think they'd be the last person to screw you over only to discover in the end...PPG is generally a good brand of paint.
As a professional, you'll find that you will be much better off if you decide to work with one or two brands of quality paints and a few products within those brands rather than having the customer dictate what you'll be using. You'll have a much better idea of the type of performance you can expect which will allow you to give more accurate bids and, correspondingly, achieve higher profits.
The only time I use the customer's paint is if it's a rescue job where they've started and need someone to bail them out. And I rarely do any of those any more - they are too much of a PITA.