Get any dye from a merchant for free
If you go up to a merchant that allows you to dye your items, click the "apply to all". Then, instead of hitting cancel or the 'x' button at the top right, hit escape, and your character model will stay with those colors. I have found out that other people cannot see this color change, and that it disappears after any loading screen.
also works by hitting the character button. you can apply this to any piece one at a time or all at once. i've had instances where it stayed with some pieces and others could view my dye. if you hit the character button after you've done this and hit the X at the top then it disappears. however if you hit the character button again (out of dye screen completely) then it'll stay.