Just wondered if there is a shelf life to icing colors. I've had some of the past colors for like 10 years and have barely used them and I've had the liquid kind for about 5 or less years.
If it's bad, you'll just know that it is. (I wouldn't worry about liquid... it's probably good forever.)
If your paste has separated with liquid on top and globs or specks that won't mix with the liquid--it's bad. Even if you break the seal on a new "old" jar and you can see that it's flaky, stringy, lumpy, or too dehydrated--it's bad. Rather than trying to remix it to save it, you're better off throwing it out and buying new ones because it's not worth the trouble if you ruin your icing with tiny specks that you thought would dissolve.