Just to add to Demagole's note, you do need to start an interview (have a model in the room) as the interface will not show otherwise. I think its best to have the model naked to get used to it, but its up to you.
Press N as Demagole has said, you can then use the tick box in the User Interface that comes up, on a particular garment and then create a colour using the RGB sliders and apply it. You can also also change the material types as well using the button and apply as well.
This should get you started.
There are a few bugs where for example, you may have the model naked, and have all items unticked in the UI but some clothes wont remove, this is compounded by not being able to ask the model to remove the clothing as its not a listed item there either.
To get around it, I now use MM to re-randomise the clothes and remove them all manually and start over, without leaving the room ect.
Im sure the little bugs will be addressed at some point, but this is exactly what I was asking for ages ago.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)