Go to your nearest toy/model shop and buy a small pot of Humbrol enamel (yep – the stuff your dad used to paint his airfix kits with!), plus some small brushes (zero or double zero size) to go with it. If you can’t find the exact colour, mix two colours and test somewhere it won’t be noticed (like under the bottom bracket) until you get a match. Build it up in 2-3 thin layers.
Get some satin black while you’re about it, as its great for covering scratches on bars/wheel rims/cranks etc.
Of course it won’t look perfect, but you’ll learn to live with it after a while.