I don't know if I am able to communicate this properly. Hopefully others will give their two cents too. lol Everyone has two cents, right? hahahhaa I've read that some take the leftover and put it in a separate container then when a color is needed they use some of it to make the desired color. Now this would mean knowledge of the color wheel. Looking in the air like a kindygarten yung un - uhhhh! Yes, I drift off... lol Trying to remember what was taught for the color wheel, of course, years gone by new colors bloomed/were created.
You could use a toothpick/scribe tool/tip of knife to plop down a beads of chosen color. All that you do is just set aside a little icing, color it, loosen it and use it. No waste.