Each time I try and colour buttercream icing black and then thin it out it seems to look like it bleeds out and goes a little purplely/pinkish. Hopefully that makes sense? and it happens a lot when I make a FBCT using the black for the outline. Any ideas to help stop the bleeding? Maybe I'm using too much water to thin the icing out?
Thanks so much for your advice.

Why are you thinning it out? Yes, it does sound like you are using too much water.
If you make your icing color(s) ahead of time.....like the night before there will be less chance of it bleeding.
Also an easy way to make black is to save all those tiny dibs and dabs of icing - that plug of icing in the tip and what you scrape from the bag -less than 1/4C - keep in a jar, in the fzr. When you need black, take it out; stir it up (yeah, it will be a funky, yucky color) then add your black. Much less color will be needed so less chance of bleeding.

Try thinning using corn syrup instead of water. That will help as well.

Great idea, thanks so much. I have been making the black and using it right away to decorate with so maybe that's my problem. My reason for thinning out the icing was to get it to the 'thin' consistency from my medium - still maybe I'm just adding a little too much water.
I'm going to save the little bits and freeze away!
Thanks again from the newbie.