I've found it difficult to match existing paint when doing touch ups on outboards or legs and have gotten the best result by repainting and entire section that is scratched, or even the entire leg. I've used Tri-lux paint in an aerosol can with good results, and learned that a good primer is necessary for it to last. Interlux makes a primer...here's what they have to say in their materials:
"Interlux Outboard and Outdrive Aerosol Primer is formulated for use on underwater areas of aluminum outdrives and outboards, prior to application of Tri-Lux II, Tri-Lux II Aerosol or Micron 33 Aerosol.
Outboard and Outdrive Aerosol Primer is a non bleeding, anticorrosive primer that provides an excellent barrier on metal surfaces, reducing galvanic corrosion."