there's been some research published on colour and therapy, so you might wants to look at that and decorate according to what your type of therapy is
pink is suggested for Police cells as it calms the detainees
green is relaxing, but too much may be too relaxing and prevent learning and problem solving
yellow makes the mind more open and receptive to intellectual challenge
orange stimulates appetite - could be a problem if working with eating disorders
maybe decorate with a very neutral base and use colour filters on lights to create an environment suited to the person's need and where they are in their therapy
avoid any reflective surfaces- no-one wants to see themselves 'ugly cry',
and if they catch a glimpse of themselves when they enter the room, they might live up to the outward image, that they project to others, instead of opening up and exploring whatever has brought them to your service
use soft textures- these are soothing both to the eye and touch, and help the person feel safe in your space