Having been a professional painter in a previous life I'll share my experience...
Auto paint colors are made up of a combination of pigment colors and metallic sizes including pearls. The first challenge is the factory standard. Today, the average paint code has between three to seven alternates (varying shades of color, metal flake, flop/shift etc) that are worth formulating. There is actually more but the auto paint manufacturers have narrowed them to down to keep the databases simple to use.
Most car manufacturers have three major paint suppliers. The manufacturer decides on a standard color for production and submits a painted sample/chip to their suppliers. The paint manufacturer then produces a formula for the standard sample and is allowed a tolerance of plus or minus 5% when they deliver the paint.
This is the first problem because the plant in the east coast may be getting a 5% shade greener on a blue metallic standard and the plant in the west coast may be getting a 5% shade violet on the same blue metallic standard. When compared side by side, they look like a completely different color. This is the reason the paint manufacturers usually have the standard formula followed by two alternates. If the alternates are not available, the painter in the body shop usually mixes the standard formula and tints it accordingly. Again, this is where a good experienced painter sets himself apart. I've seen a good painter really struggle to butt match white panels and get a blend-able formula straight out of the paint code for a complex pearl toner... Caffeine and alcohol have an negative effect on the eye that can impede seeing certain tints. The amount of red tint is a paint formula for example can elude a painter if he drank a lot of alcohol and caffeine within the last 12-24 hrs. More so if he's drinking and mixing formulas
The second reason for variances in paint colors is the metallic color applications. The metallic colors are now classified in 7 categories. Extra fine, fine, medium, medium coarse, coarse, and extra coarse. The metallic colors control the value (lightness and darkness) of the color similar to what white does in a pastel color. This can include 'pearl' flake (powder or liquid versions) that are now so prevalent in modern factory automobile paint finishes. To much pearl and you wash out the formula...
Metallic colors will cause variances in color when applied. Temperature, paint film thickness, flash off time between coats, fluid tip sizes, speed of the spray gun, surface type (Plastic or Metal) and humidity will all cause the color to shift lighter or darker.
The rule of thumb: the longer it takes to dry, the darker the color will change as it dries. This is caused by pigment flotation. The metallic flakes will settle down to the bottom of the paint film and push the pigment up causing the color to shift darker.
The reasons above only mention the variables at the car manufacturers level. So what happens to a color after three years of sunshine? Many people think that colors do not change, but they do as most of the guys here know. If you own a car that is at least three years old and has been out in the sun most of the time, remove a pinstripe and you will see the original color. Removing the stone guards on a Guards Red 911 is another example...
The sunlight has ultra-violet which has absorbed some of the pigments. Blue metallic colors sometimes shift to a greener shade, and reds will turn pinkish or more orange. The auto body painter has to deal with matching an oxidized color in addition to new OEM colors. The new paint to be applied will look brighter and cleaner but the rest of the car looks dead even if you polish it. Again, a great painter knows which toners to ad to the formula to get a 'dead' match...
Auto Body Shops today have a greater challenge than just color match or water based paint. As mentioned above the texture or (Orange Peel) also has to match the original finish in order for it to look pre-accident condition. This can be accomplished by using the proper spray gun, polishing equipment and most importantly in my opinion, experience.
A great painter knows just how much peel to cut with sanding to match the original panel. Porsche for instance has a good amount of orange peel. If a painter tries to deliver the smoothest finish on the panel he paints it won't match the adjoining panels level of peel.
Now back OT:
Black:
Blue Black (cool and deep) Red Black (rich and deep) Yellow Black (warm)
All black tones contain other colors (toners) in their formula. For instance one of the truest/simplest forms of black is GM (99/8555) which contains a little blue (for a darker deeper cooler tone), red (works with the blue for a deep liquid tone and several others. These additional toners are minuscule in ratio to the main toner which is "black", but they are there.
White: White is very similar in that there are cooler whites, containing transparent blue toner and a little black toner (Think Honda, Porsche GP), warmer whites containing red oxide, yellow oxide (Ford), and on and on...
A good professional paint supplier has a tool (chroma-scan) This is a process where they use a hand held scanner that analyzes the paint chip, and then looks up in a database for a currently produced automotive color that is closest to it. Then it lists out the toners that need to be added to match it exactly...
A good painter can take the original formula and ad toners to "butt match" a panel. This comes from years of experience. Another good trait of a real pro painter is matching aged paint, think a 60's or 70's single stage metallic paint that no longer has luster. An excellent painter can match the metal flake, flop (color shift or the difference of surface tone based on the angle of view) and gloss level or lack of...
