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 (
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, picture below) are just what you need. These are the smallest packages of premixed grout I've seen and they go for $3-$5, depending on where you buy. Other manufacturers, like Laticrete, make pre-mixed tile grout, to but the packages are (much) larger.
2.25 fl. oz is said to cover 3 sq. ft, which I'd be very skeptical about. However, if you need it for patches and repairs, this should be plenty. They also make 5 fl. oz ones, slightly more expensive. It sets in about 1 hr when applied and once you open the tube, you get a few months until is will set in the tube, too. So get the smallest package you think you may need and just use it all up in one application. Next time order a fresh one.