There is no "open source" because it is a shitton of work someone has to pay for it and actively manage, and open source means anyone else could just take your income source (selling the paint catalogs) and re-make it cheaper.

If you're buying a lot you can just design the color, get a bunch of samples mixed and pick one.

Hell, you could go to good paint shop and get anything you want matched and get a recipe for it, that's how car can be painted to match the paint that changed the color with age.

Just costly for anything not mass market.

A paint shop does not perform the testing required to confirm a paint meets the classes of specifications the GP of your comment describes.

Achieving that level of known properties requires equipment, personnel, and materials beyond what would make commercial sense outside of the market segment in which the cost of Pantone systems make sense.

To put it another way, cost of entry is Pantone’s moat and why it has been filling the same market segment for many decades…where $21/month is pretty much no different than free.

Also a paint shop does not do that for free. I remember going to one and while talking to the owner his subordinate kept walking up and asking if this is the right color (they were matching) and the owner said, I think it needs a bit more ... 5 minutes later he would come back and show him and keep iterating. I don't think you can always just put some paint sample under a light and have a computer spit out RGB. It takes time and IS expensive to match paint, especially automotive, etc..

It would be an appropriate undertaking for something like NIST, I would suspect.