I'd like to create a small override mod for the pie menu. There's something that's been bugging me lately about it and I'd like to recolor parts of it. Does anyone know where I could find the relevant IMAG resources in the game files? Also, how do I create a mod to override them, after I recolor those images? Do I just dump them into the package file and make sure they have the same instance IDs as the files I'm trying to replace?

A lot of the UI elements are found in DeltaBuild0 and FullBuild0 (check them in that order).
You can search in S3PE by entering a keyword into the 'Name' field, checking the box, clicking 'Set', then checking the 'Filter Active' box. The HUDPieMenu images may be what you're looking for.

You can select the resources you need and Export > to Package, which will prompt you to name and save a new package.
After you've edited the images, you can simply right click them in that package and choose 'Replace' to swap in your edit. Save, and the file can be dropped into your Packages folder.

Some parts of the UI are also controlled by LAYO resources, which are text files with styling information.
If you can't find what you want to modify as an image, it may be helpful to download an existing recolour mod and narrow down the contents to what affects the part you need.