Upon opening the file, you'll be presented with a few sample colors. Check how these are made and experiment.

Once you're ready, import one of these materials into your own project by having your project open and going to File, Append, navigating into this blend file, Materials.

Optionally, you may want to import the compositor nodes if you'll be using Intel's Denoiser node.

Base Color and Pearl Color:

Base Color will control the main color you see when facing the surface.

Pearl Color will control the main color you see when surfaces are at an angle.

Flake Color Mismatch:

Flake Color Mismatch helps dark colors get a more intense "glow". It creates a hue variation that shifts away from the base colors. It is best left at 0.000 or at low values of up to around 0.250.

Pearl Layer Thickness:

This is the coverage of the pearl layer. The higher the number, the more of the pearl color will be visible towards the center. A value of 0.000 is 0% of the pearl layer visible, while 1.000 is 100% coverage.

Highlight Intensity and Spread:

A sheen applied to give paints a more "raw metal" look. It cuts down on deep colors.

Pearl Roughness:

The reflectivity of flakes in the pearlescent layer. 

Low values (towards 0.000) will make flakes seen at an angle more noticeable, as in, they will pop out as bright pixels. 

Increasing the value will fade this effect and smooth out the highlights.

Flake Scale:

The scale of the flakes in relation to the car/object. The default value has been scaled as approximately real life size, however, if your object doesn't respect real world scale this is a way to quickly adjust the scale of the paint.

Clearcoat Roughness;

The roughness of the clearcoat layer. Gloss and matte clearcoats are set here.

Candy Strength and Brightness:

Candy effect. It darkens the paint and creates harsh contrast zones. It's best used with moderation for enhancing deep colors.

If used in conjunction with brightness, it can be used for the opposite effect, to light up a color.

Metallic Flakes:

IOR of the metallic flakes. Results from altering this are negligible, only noticed as a very small contrast change from far away. It's best left ignored unless you input a value lower than 1.000, which I don't exactly recommend.

Metallic Roughness Range:

The reflectivity range between flakes, where minimum and maximum values are set. 

The more set apart the two values are, the stronger the contrast between flakes. 

If both set to the same value, all flakes will have the same level of reflectivity. 

The overall reflectivity of the paint, when seen from afar, is the average between the values. 

The min and max can also have the values inverted to give an alternate shading effect.

Pearl Metallic Flakes Strength:

The metalness of the pearlescent layer's flakes.

Flake Rugosity:

The rugosity of the base paint coat. It's a bump map. It will alter how light bounces off the paint surface in close-ups. Small values up to 1.000 are best. The effect is negligible from afar.

Clearcoat Orange Peel:

The "orange peel" texture of the clearcoat. It controls how smooth the world reflections will be.

Hue Shift:

It's best used in very small quantities. It shifts the color hue of the flakes. It can also be used as one of the ways to quickly create an iridescent effect.

If the Range is set at 0.000 and the Offset at 0.500, the result is the same as having the effect off (at 0.000). 

The range, a 0 to 1 value, dictates the range of colors in the hue the paint shifts through.

The offset, a value from 0 to 1 where 0.5 is "the center value", offsets the base hue the range acts upon.

The Pearl Hue Shift Effect is by how much the hue shift effect affects the pearlescent layer.

Close-Up Metallic Flakes Strength and Color:

These flakes are mostly visible in a close-up, the effect on color when seen from afar is almost negligible. 

When the effect is at 0.000, these flakes are slightly shifted in hue and brightness relative to the base color. The Effect value overrides the default color to show red, green and/or blue flakes. This is useful to give colored flakes to paints low on saturation, like black or white paints.

The color controls the strength of the red, green and blue flakes individually.

Updates:

This shader won't be updated (unless there's some bugfix to make), however, a new shader for the same purpose, but using more efficient methods, is at the moment in testing phase. Depending on the sales success of this one, it's likely this will get a price drop and buyers of this shader will get the new version for free, once it is released. If you're a buyer of this shader, and want to try the new version (undocumented), you can request it through product support ("About" tab - Ask a Question).