You can get tiny brushes at a hardware store (d/k about HD) or art supply store. Premixed patching compound is for sure available there. Use an x-Acto knife with the blade reversed to push a little blob of compound into the hole.

Color match — if you know what paint you used initially, you should be able to get a batch that’s very close in color mixed up. And next time you repaint, store a gallon or so of paint in bottled water bottles. The caps seal well so the paint’s ability to dry is minimized compared to a can. Should last quite a few years for touch-up purposes.