Where can I find touch-up paint? I need grey touch-up paint for the headtube on a 2014 Whyte T-129s. How can I find out which colour exactly to buy?
To sum up some comments:


Contact the dealer or manufacturer for a specific color code or name
Search various paints based on a similar code
Get some touch-up paint from an auto parts store and use that


I would add that if you just go with regular touch-up paint, to make sure the gloss level matches the bike (most bikes have a high-gloss clear coat to protect the finish).

Also reference automotive touch-up guides to get an idea of how to best apply any paint (surface prep, etc).