I am just starting to dabble in xml tune modding (and I don't know if this pertains to xml/itun files) but does anyone know where to remove or change the colour for the extra trait slots? They annoy me so much XD I'd like no outline or the default outline UI blue.

You can learn a fair bit about editing UI elements from the thread about UI recolours: https://modthesims.info/t/553732

The UI is made up of IMAG textures and LAYO xmls, the most difficult part mostly being locating the file you need.
Most of them are named, but whether EA's naming conventions point you in the right direction can vary.
You may be able to narrow your search by trying out a few of the existing UI recolours out there and seeing if any of them change those trait outlines- because if they do, you at least know the right file is in their package vs combing the game files in their entirety. S3PE can preview image, so you can pan through the IMAG until you find what you're looking for

Thanks! I'll look into it

If anyone wants to know the files are from EA DeltaBuild0 package file:

• ATLAS_HUDSimologyPanelEntry_00
• ATLAS_CASTraitsTooltip_00
• ATLAS_CASTraitsTooltip_01

Also I found for the trait selection pop up file:

• ATLAS_TraitsPicker_00