A while ago I came across a website that did touchup pens for a catalogue of old bikes, but I can't seem to find it now. I have a specific old bike that I want to check if they have, before I go to the auto-shop.
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It may help if you state the model and year of bike you have. If you take the bike into a shop that sells paint, they may have a color matcher (eg a Pantone) which you can aim at the bike and get a color code. But in all likelihood, the paint touchup pen you use will be automotive (just getting the color right is the hard part). – Batman Mar 27 '16 at 5:46
Gunnar bikes have a selection of touch up paint for classic bikes.
They also have great information on how to use it, and what results can be realistically expected. :)
http://gunnarbikes.com/site/order/touchup/
ColorRite also has a wide range of paint touch up pens available.
http://www.colorrite.com/product/touchup-pen-primer-1071.cfm