You can make additional custom recolors of paintings that do not override the original using S4Studio. You would need to install the program though and have some kind of photo editing software such as Photoshop or Gimp. Here's a YT video explaining the process if you want to check it out:
The Sims 4 Tutorial: How to make your own Custom Painting
https://www.youtube.com/watch?v=pX_Uu6YE4d8
Hope this is what you're looking for, good luck and happy Simming! *Hugs* :)
EDIT: Oh, whoops, I apologize, you wanted to actually have your Sim paint custom paintings, this won't help you with that. Sorry... o.O
