When you have an firework star with an color in it you can dupe it when you try to add an extra color.

Look at the screenshots how to reproduce it.

I`ve made an video about this bug: http://www.youtube.com/watch?v=GDLTMBarUis&feature=youtu.be

Thanks to Kahless for this nice 'Steps to reproduce' list!

Steps to reproduce:
1: Craft at least 2 stars (gunpowder + dye). This can be done independently or together.

2: Place a group of stars together into a grid with a dye (can be new color or original color).

3: Note that the output is the number of stars that are in the group. This is where the bug is.

That is to say, when you take 1 group from the output (by left clicking), you will receive 2 new stars, and it will remove 1 dye and only 1 star from the grid, leaving the remaining stars in the group still in the grid.
As a result, if you started out with 2 stars, you will now have 2 new stars + 1 old star = 3 stars. If you start out with a stack (64) of stars, you will receive 64 new stars + 63 old stars = 127 stars.

Expected result: When crafting a star with a dye, the output should be 1.
Observed result: The output is actually the number of stars that were in the group.
Note: Adding more stars to the grid group will not alter the output number. To update the output number, remove and replace the group (left click on it to remove it, then left click again to drop it back in).