Media item colour is determined by an underlying defined colour (either from the theme by default, or a custom colour is you have set one) and a image over the top of it, which may or may not have transparency to show the underlying colour. The images are item_bg.png and item_bg_sel.png (for selected items).
Reaper has a legacy colouring method, and a more modern 'V5' colouring method ...
read this if you're interested. Changing tinttcp will sort-of work to regress the colouring method to the legacy mode by reactivating the disabled options, but in an unpredictable way. The correct method is to remove the 'version 5' and 'tinttcp' lines entirely.
But I'm pretty sure this isn't what you want to do; you want to remove the item_bg images.