I'll try to not make this a novel.

There is more than one way to make a paint color. Different combinations of colorants can result in a color that appears identical to an "original" sample.

There is a scale of acceptable difference between two colors called Delta E. Every color "match" can be qualified against this scale:

Delta E value Meaning
0- 1 A normally invisible difference
1 - 2 Very small difference, only obvious to a trained eye
2 - 3.5 Medium difference, also obvious to an untrained eye
3.5 - 5 An obvious difference
> 6 A very obvious difference

Every color can be measured with a device, like a spectrophotometer. The result you get from measuring the color is called a spectral reflectance curve. It's a breakdown of the color by wavelength of energy. (wavelength in the visible color spectrum)

Another way to think about is, a color's spectral reflectance curve is a lot like your fingerprint. It's a unique identifier to each color.

You can use different colorants to create colors with almost identical spectral reflectance curves which means the colors all appear to "match" - despite the fact that each is indeed unique.

Sticking to a single, original color formula is not guaranteed to happen even within the same brand. For example, it is possible the formula for XYZ Paint Color will be different for different quantities of paint, i.e. a different formula for a Color To Go jug, a different formula for a quart, a different formula for a gallon.

Some colors are simply easier to copy from brand to brand, quantity to quantity.

Some colors are not so easy. In fact, every time a manufacturer updates their bases or colorants, there is a risk that some of the colors in their palettes can no longer be mixed. That happened with Sherwin Williams the last time they changed their paint recipe - there are colors in the SW palette that can no longer be made to match the original chip, there are stain colors from Pratt & Lambert that can no longer be made, etc.

Without a doubt buying the color from its home brand is the easiest thing to do.

But it is possible for the color to be matched in a different brand, using different 'ingredients'.

The problem is color matching using different ingredients is hard. You can not color by numbers and computers alone. Color matching/mixing is a specialized skill set that combines color math, computerized aids, and a professionally trained eye. The color mixer person must understand the 'ingredients' they're working with extremely well. Which usually means, that color mixing/matching is the job they do every day, all day.

The problem is not so much the unique ingredients each paint brand has. It's more about the lack of a skilled workforce of colorists.

But that's a long story to tell. And paint manufacturers most certainly do not want to admit that they lack anything, especially skilled colorists.

So, instead of telling it like it is, and/or dealing with the reality of finding the right people who know how to mix/match color, it's much easier and shorter to just say you have to buy your paint color mixed in its home brand.

Full spectrum paint colors are often part of this conversation as well. Because even tho it may be possible to mix up something that appears to be a color "match" to a full spectrum paint chip, the energy is not matched.

Full spectrum paint colors are mixed using a representational amount of colorants from each spectral hue. Which means their spectral reflectance curves, their measured wavelength energy, should be full and balanced across the entire visible spectrum.

And, of course, this can lead to bigger conversations about 'regular' colors too. Like, if one is very sensitive to the energy color brings to a space. Something to ponder is, it's possible one can be attracted to the energy from a color mixed in its home brand and completely disappointed by how an attempted color match using another brand's ingredients not only looks but how it *FEELS* -- despite the fact that the color match is qualified as a "match" per that handy dandy Delta E chart thingy.