Below are 4 variables that also effect the way a color match will turn out. 

Color Gloss is the level of reflection a color gives off at a 60 degree angle. The level of gloss is measured in % with a gloss meter with 100% being the shiniest and 0% being the dullest. Here are some common terms used when deciding gloss level: 

High Gloss 85+%
Full Gloss or Glossy 75-85%
Semi Gloss 50-75%
Satin 30-50%
Matte or Eggshell 20-30%
Flat 0-20%

Metallic colors are one of the hardest colors to match. This is mainly because the metallic flakes will cause the actual color to appear light or darker depending on the angle the metallic flakes lay, what angle you look at the finish, and how the light is reflecting off the finish. This is why touch up paint manufactured for a powder coated metallic finish will not match at certain angles but it is the best possible match without having to redo the entire piece.

Surface Substrate
The surface that you are painting on can have an impact on the outcome of a color match. This can be broken down into 2 different categories.

Surface Color
The color of the surface that you are painting on will have an impact on the color. This is commonly known as the bleed threw effect. If the surface you are painting on is darker than the paint you are applying, this will cause the paint to appear darker than it actually is. This would be opposite if you were to paint on a lighter surface.

Surface Texture
Textured finishes can cause a color to appear duller and darker compared to having a traditional smooth glossy finish. This is cause by light refracting off the textured surface giving a shadow like effect. Touch up paint can not simulate this effect in a aerosol can without having a 2 part system. This makes Surface Texture the 2nd hardest factor when color matching paint. 

Film Thickness
Film Thickness is the amount of paint that separates the surface substrate to the outside finish layer of paint. This layer is measured by a gauge using Mil thickness. It is recommended to have a good Mil thickness to help separate the earths natural elements water and oxygen from reaching the unprotected metal. Film Thickness can also be tied back to surface color and gloss level. In order to completely remove bleed threw, you need a good 1-2mil film thickness to combat this effect. When using a paint that has a gloss level under <85%, multiple coats can increase the gloss level by upwards of 10%+.