I am icing a cake with white buttercream icing. If I write using black buttercream, will it bleed if I refrigerate it overnight?

Is there reason you are refrigerating it? If you're not using perishable fillings, then I wouldn't refrigerate it. The condensation from the temp changes could cause your colors to bleed.

Use either meringue powder in your butter cream or milk instead of water in your butter cream. Both things will help your colors from bleeding. Then don't store it in the fridge if you don't have to.

I find that sometimes if my black gel is getting old...the purple base tends to really show and come out.I find this happens to if I leave icing in my spare fridge and use it over and over again so I try and make black fresh everytime...

I was going to refrigerate it because I used milk in the icing. I did use meringue powder in the icing. Would it be safe to leave it out until tomorrow afternoon?

You can leave it out. The amount of milk and sugar makes it safe to leave out, some sort of chemical reaction. There is a thread on it here if you search for it, which I just did and cannot find it.

Here is the answer to your question from the baking 911 site.

Question: Why doesn't buttercream made with butter, powdered sugar and a small amount of milk need refrigeration?
Answer: Because of its high content of sugar and fat. Micro-organisms need water to grow. When the sugar content is so high, the sugar binds the water in such a way that micro-organisms cannot utilize it. Technically its called "water activity control."

Thanks!

It's a fact, dark colors are going to bleed. Especially overnight. Adding meringue powder to your bc icing will slow the process. But, it may be mute, depending on the environment where you live and the conditions of the reception.

Is there anyway you can add the stripe at the last possible moment when assembling the cake at the venue? Have you considered black satin ribbon?

Thin fondant circles would certainly look the best. Tell the bride as the fondant sits on buttercream it will be soft and hardly noticeable to her guest. The circles will have a more exactness in the finished look than will bc.

Good luck!

My only disaster with black icing piped onto white was when my cake didn't go into the refrigerator. I kept the cake in a very cool (but not as cool as my refrigerator) place in my home and the next morning, the black had run down the cake and it was a wreck. I always use meringue powder but that didn't help this time. I have never left another cake out unrefrigerated since then and I haven't had the bleeding issue happen again either. I know there are a lot of mixed opinions on this but for me, my cakes go in the refrigerator!