I used this program years ago to recolor some posters. Today I wanted to add some new recolors and I got this: "Sorry, the Object you selected can't be recolored (yet)". Last time I used it was 5ish years ago. Is there a way to fix this or a similar easy program for recoloring?

You can use the Object Workshop in SimPE, and you also need a photo editing program for the recoloring.

Tutorial here (mostly for the SimPE bits, but it briefly shows some tips for Photoshop/editing, too) - https://poppet-sims.tumblr.com/post/54039893647.

Make sure you have the CEP installed, and that everything is properly linked up in SimPE.

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If you want to use the WoSPE, just adding in a tutorial (which is a bit old, so it's likely the setup has changed), in case there are steps you've forgotten.
https://modthesims.info/t/239197
I have no idea if this one works with the newer versions of TS2 (The UC/legacy/etc.) since they've got a different setup than the old CD versions. Depends on whether it was updated, and when it was updated last.
You probably need to make sure the CEP and all that is properly set up, too.