Can you change IntelliJ colors on per project basis?

Another possible solution might be to create different scopes for your projects and apply custom colors. Note: I see this more as a workaround because (IMHO) scopes should be mainly used 'in-project'. The downside with this solution is that you have also to handle the build-in scopes like Test.

My solution is based on the above - where I created a 1px x 1px coloured png and used it as the background image at.. Preferences > Appearance & Behaviour > Appearance and there's a button 'Background image' .. But .. I found that resizing my project window caused a huge lag in refreshing it to the new size. .. So .. I tried with a 200px x 200px image and resizing now works as normal. Make sure you also tick the 'This project only' option.

You may want to set a different background picture for each project to distinguish them from each other. This could also be a single solid color picture for example. Enter Cmd+Shift+A and search for the Set Background Image action (Ctrl+Shift+A on Windows).