Object editing newbie here, but I can photoshop a little. I want to make an invisible recolor of the spandrel of a staircase (for personal use) so I can place the stairs on top of each other without clipping. How does that work with Homecrafter?

For this type of setup I think you'll need a staircase that's open underneath. You'll have to download some because I'm not sure if any of the EAxis stairs are open.

If you have the UC, here's how to make them work properly (there's an issue with getting them to work - shouldn't be a problem if you have CDs though). There's also a few links.
https://www.youtube.com/watch?v=8MmKiDiPnwg

The staircase is custom, which I got from here: http://www.simlogical.com/sl/Sims2P...irs_Winders.htm . So I basically wonder how I can make the staircase open by making the spandrel invisible

Maybe try an invisible cc wall. https://modthesims.info/d/59793/cus...-amp-index.html

You may need modular stairs that are open underneath. The ones at Simlogical aren't, but the ones shown in the video are.

The reason why I use the Simlogical one is that it has a 90° shape (so a U-shaped staircase instead of straight, which is more practical in smaller hallways), so that's why I wondered how I could create a recolor of it with an open underneath.

Those stairs have the wall as a separate subset from the treads/railings, try recoloring the wall with a transparent texture.