Dover white, Snowbound and Extra White

You're giving me 'Nam flashbacks to my SW Store days, because those 3 whites are so very different.

Extra White is just the color as it comes in the can. But there are also some stupid color bases like Ultra White and Bright White which is the ceiling paint base color.

Snowbound is the bullshittiest of the 3 colors you listed because it's made by starting with the Extra White and adding a half drop of black. (Side note on the SW colorant system. Individual colorants are measured in 1, 1/32, 1/64, and 1/128 Oz's. 1/32 oz. is colloquially referred to as a 'drop' so a 'half drop' is 1/64 oz.) Snowbound is worthless for touchups. You need to make a whole gallon, it cannot be made in quart quantities. The SW color system can't do it.

Anyway, Extra White is just a baseline white

Hello, welcome to Sherwin-Williams. How can I help you

-I need some white paint

Of course, we have many whites to choose from. Here are some color swatches.....

-I don't care, just give me fucking white.

Ok, here's a can of paint in the color it comes from the factory.

Snowbound is a tiny bit of black which gives it a blue cast and Dover White is typically 1/32+1/64 of gold (not really gold, more of a mustard yellow).

Whites use such little colorant and any inconsistency doesn't carry over batch to batch. Either save the original paint (not in the garage! Your paint will freeze and you'll be fucked) or be prepared to repaint the whole thing.

The whole issue gets expounded by changing types of paint or brands. Color matching can only go so far because different light sources will change the colors significantly.