You can have the original can of paint and still the color will be different. That is because the little metallic particles will lay out differently each time giving it a more or less silvery appearance each time.

This is due to to differences in humidity, air pressure, spray gun, technique, etc. These things can never be duplicated.

Perhaps you have seen it, and now you will be looking, where the paint match is perfect when looking at it straight on, but looks terrible when viewed from another angle, or under different lighting conditions.

There are other factors. Most of the paint manufacturers will give out around five variations in the paint code. These are factory variations! There is not the consistency you would think. I have no idea why they do that. I have heard they bid out the materials in some cases. They may use a different brand of paint. I understand some paints are water-based, but there is no useable water product available for refinishing. Some day water paint will be universal. Emissions and all. Then they will sell us $20/liter water with magic properties. Count on it.

Here is something most people don't know: there is no factory paint available for refinishing. In the past one could order paint from their BMW dealer, but no more. EPA factors, hazardous shipping rules, etc. are blamed. I always thought is was because it didn't match anyway, so why bother.

The bigger and better shops have their own paint mixing systems. There is a big rack of color all being churned by a system of chains and paddles. Many cans of different sized and colored metallic particles, pearls, toners, etc.

All this is mixed by weight on a scale that can measure a tiny amount. One coin weighs more than another, depending on wear, for example. Fun to play with.

The painter test-sprays panels that are held up to the car and then tints it. Our painter, a gem, could do this accurately. It is a gift. A painter can save a shop's butt, or create havoc. The paint companies say it is science, but it is still art, even today.

Our shop had two such systems as one brand would match certain colors better than the other. We used European brands as they seemed better for the work we did.

There are also many different primers for different uses. Non of these are the old grey primers many of us grew up on.

BTW, two-step paint refers to color and then a clear coat over it. Three-step means there is a pearl prior to the clear. Solid is for those no-extra-cost Alpine whites and black. Sometimes the manufacturers put a clear-coat over those for extra scratch protection.

You asked about fade due to the elements. Today's paints are great about not fading. The costs are hugely more than they used to be, but there is no question that they are so much better. Fade is not much of a factor except for the solid yellows and reds. Those are very hard to match even when they are new and fade in funny ways after that.

There is more, but I am going fishing now.