Inside the booth
Use one colour accent in a portrait strip
Let a small detail connect the frames.
A scarf, card or background detail can become a recurring accent without taking over the portrait.
Choose a comfortable detail
Use something you already have and enjoy wearing or holding. Keep it away from the face and equipment. Test it under the actual lighting; a colour that looks strong in the room may appear differently in the capture. Avoid changing the entire wardrobe just to satisfy a visual rule.
Compare the intended output
Check the sequence in colour and, if relevant, in monochrome. The accent may lose its role in black and white, so judge the version you will actually share. Keep the original capture and make treatment changes on a copy.