I've developed a custom content creation tool for The Sims 4 that I named #Caster, after CASt. The tool allows you to select a base texture and apply a pattern on top of it. Advanced users can use a "Mask" and apply up to four patterns in different areas of the image. There is also a built-in Image Editor for changing the brightness, contrast and tint of any part of the creation.The program can read package files, but it cannot write them.
Due to a serious bug, the tool can no longer read package files. You will still need Color Magic and S4PE to actually import content to the game.
You can download here:
The official page for the tool is here: (sorry it's in a mess at the moment after upgrading from the first version, I'll clean it up another time)