The nice thing about paint is that you can almost always re-do it!
In response to using the customer's paint:
I am usually agreeable to use whatever the customer insists on using, but communicate with them that they are not the paints I typically use, and therefore I am not as familiar with how they handle, or any idiosyncrasies they may exhibit, etc...
I also disclaim any liability associated with using paints that I am not familiar; That includes extra time and material that may be the result. I think this is a fair and reasonable policy, and the customer who insists on using their own paint, will likely not be surprised by it.
In response to using the customer's paint:
I am usually agreeable to use whatever the customer insists on using, but communicate with them that they are not the paints I typically use, and therefore I am not as familiar with how they handle, or any idiosyncrasies they may exhibit, etc...
I also disclaim any liability associated with using paints that I am not familiar; That includes extra time and material that may be the result. I think this is a fair and reasonable policy, and the customer who insists on using their own paint, will likely not be surprised by it.