Those little pre-mixed tile grout squeeze tubes are actually perfect for patching and repair work. It's really hard to measure and mix parts properly when all you need is just a couple of ounces of the stuff and the factory pre-mix has just the right ratios and is ready to go. There's one limitation though: the premixed grout that's very widely available is bright white. If you need different colors, you'll have to tint them with acrylic paint (most products of this type are acrylic based) and there you'll have the issue of measuring and mixing tiny amounts once again, especially if you want repeatable results.
Anyhow, if your grout is white, the small squeeze tubes such as Red Devil's 2.25 fl. oz ones (