Why are people so unhelpful around here?
OP, remove the cover and take it to a good paint shop. They will match it more or less perfectly, then you paint the whole cover if you want a first class job. Most paint shops will mix paint and put it in an aerosol can if you havent got the gear, blending the missing paint in is hard to do well so unless you just want your eye to not focus on the blemish I wouldnt bother touching it up. Ask the paint guys for the right kind of paint, they should give good advice if they are professionals. The key is good preparation, removing all oil residue by repeatedly cleaning with degreaser and/or acetone. Some people even recommend warming the cover to help lift oil out of the pores, then you need an etch primer, light coats and job done. G'luck.
