system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so hard to match white paint?

I went to Home Depot the other day to get some paint to touch up a white wall. Even though I had the information for the custom paint blend (Sherwin Williams), they said they couldn’t guarantee they’d be able to match it and white paint was notoriously hard to match. Why would it be so hard to match a color as simple as white? It seems like there would be fewer variations than on a color like red or blue.
gpt
It depends on what colorants are in the mix, as well as the size of the can you’re getting the match mixed into.  

The base starts off pure white (titanium dioxide), so any color added is going to create a much more potent change than it would if it was a heavier color with lots of colorant already in there.  

If it’s a quart of paint, even the smallest possible drop of red or yellow will often be far too much because of the ratio of colorant to paint.

A really good paint mixer will say “I can do it, but you won’t have a formula to go by” and then smear the smallest drop onto their finger and just dab a partial bit of it into the paint and then check the color.  These people are extremely rare, and I’m pretty sure would NEVER exist in a corporate establishment.

If you want the best match on a white color from the lowest common denominator, get a big bucket of paint to match into.  With a much better colorant-to-paint ratio, there’s significant room to play around and the smallest drop won’t ruin your color anymore.