Molten sugar will recrystallize in a hearbeat, if but ONE TINY crystal of sugar is introduced to the mixture.
I was always taught to have a cupful of HOT water with a pastry brush next to the pan, and brush all the sugar crystals from the sides of the pan into the mass before the water boils off (you know this happens with the bubbles change pitch and frequency).
Also, some things help to keep the sugar from crystallizing. Some lime juice, just a wee little bit, but not enough to change the flavor, will work. Also, a couple tablespoons of corn syrup do wonders, since it's an "invert" sugar and naturally stays liquid.
I almost always add just a bit of corn syrup to my syrup mass before I start boiling, to help ensure that my candy doesn't crystallize on me.
If it does crystallize, there is a quick fix, and you did that without even knowing what to do. Just add some water, let it dissolve, and do it again.
Personally, I always go by temperature, not color. You can get a cheap candy thermometer for a few bucks.
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