I am made a light saber cake last night for my son's birthday today. The saber part is bright green fondant, but the handle was just to be grey buttercream (smoothed with the Viva method) and although it was a great shade of grey last night this morning it is a shade of army green. The cake was basically just awaiting the finishing touches. The grey frosting in the piping bag still looks grey, but on the cake it is green. I also have a piping bag of black frosting for detail work that was made from the same Wilton black gel color. It was not new so I think from other posts on the subject (which I only looked up after the fact), it seems like it is more prone to happen with old coloring so that will get tossed.

Party is in 4 hours with lots of other things to do. Any quick fixes or just assume that the 6 and 7 year olds won't notice? Except for the color, the cake looks super cool - it is 3' long!

 I have had the same problem with Wilton black trying to make gray.  A touch of the black makes a gray green rather than true gray.   I recently needed gray, and it happened again, so I tried a some Kroma airbrush coloring that I had.  A touch of that to white fondant made a beautiful gray.   There may not be time to find another brand.  Anything else on hand?  Do you have any luster dusts, sprinkles, or sanding sugars to cover it with?   any other brands of colorings lying around?  A touch of red or brown might neutralize the green, or you could try mixing your own black which is a combo of all colors, but that would take a little trial and error to get just right.  Bottom line, I think your 6 and 7 year olds will love it.  We are our own worse critics!  HTH

I am guessing that short of scraping off the crusted buttercream and starting over on this handle there is probably little I can do. I thought about picking up some white fondant at the store (I used store bought for the green part and don't know how to make it) and getting a new black coloring to try to make grey fondant and then covering the handle with that. What is so frustrating is that it must be in response to air/light because the grey frosting in the piping bag (which I was still going to use to pipe the edge of my now olive green handle) is still the great shade of grey. I guess, I should pipe it sooner so it has time to turn army green, too? Yes, we are our own worse critics.

How do you use luster dust? It would be cool if I could put a silver shimmer on it. I have to go to a party store to get balloons, so I can see what they have. Can you use luster dust over buttercream? How do you apply?

This is probably too late, but you could buy a small pack of black fondant (just saw some at MIchael's) and mix a bit into white to get gray.  Luster dust is usually painted on - mix it with vodka or an alcohol based extract, then brush it on. It comes in many colors, including metallics.   (the alcohol evaporates off).  I have only used it on fondant, and I don't know how well it would work on BC< sorry.  Also, the light part of light sabers come in many colors - red, green, blue, even purple I think, depending on the character.  you could really make it any color you want!

So, I found a fast fix that worked pretty well. I bought Silver spray and just sprayed the part that was supposed to be grey silver. It still had a little olive tone, but overall it had a good metal look. I did the light saber part green as per birthday boy's request. Cake went over well. Thank you for your input and help. :-)

Great fix!  Glad it went well.  

AIn the future if you have gray trouble try this. If your gray looks green add purple, and if your gray looks purple add green. This has worked great for me.