Go to a specialized paint shop, and bring a rim with you. Or drive it there.
They are usually very good at matching paints, as this is their job.
They'll either sell you a spray can for a bit of profit, or just the paint for less profit. A scuff might just be doable filling it in with a bit a clear on top.

 

Yes it's a very small scuff and would just like to touch in as I'm sure it will happen again at some so a small touch up stick would be good , so annoying!

 

If it's a Nissan rim just go to them....I bought a touch up kit for my car just the other week. About 10£ for color and clear.

I like this guy for instructions even though you probably know of him...usually makes sense:
https://www.youtube.com/watch?v=v5QAjWbx03s

 

Bought one of these last week
R35 GTR Black Edition 2009 Wheel Paint + Clear Lacquer [KH2PAINTKIT] - £9.19 : Fasteronline
Good match

 

Try Volvo Dark Grey Pearl, available from Halfords, I used it for a small kerb scuff, very good match on the 09 Premium wheels.

 

Thanks will try and get it sorted worryingly easy to scuff