Depends on what you mean by ‘Bad’.
I tend to dye large quantities of yarn or fiber at a time so I’ll mix up 1/2 gallon batches of each color. Most of the time, there will be some dye left over.
As it sits in the bottle, things happen. Some colors, like Kelly Green will develop big blobs of weirdness. Others will separate and dye particles fall to the bottom. Still other colors completely change. Olive Green, for instance. I didn’t use the left over dye for several months and it changed to a dusty blue.
Can you still use it? Sure. Just realize that it may not be the same color as the original batch you mixed up. If a dye has been sitting for more than 2 weeks, I will strain it before using. This gets out most of the odd globs and loose particles. Also, if you need a specific color for a dye lot, always mix up a fresh batch of dye.
Keep in mind, this is strictly from my personal experience using Jacquard Acid Dyes.
One more thing… if you decide not to use your left over dyes, please don’t just dump them down the drain or in the yard. Add some baking soda to neutralize the chemicals first.
Kentucky Blue Fiber Co.