The white interior paint is on sale. Do they mixed the colors
From that can?
Or do you buy a can of tint base paint and they add the colors
To that one.

I cannot remember how they did it??

The formula for mixing paint is on the paint chip you pick for the colour you want. They add tints to a can of white and then mix it well.

Quite often the paint used to mix light colors is labeled light base. How different it is from a white would be hard to say. The bases may have less white pigment in them. If you are venturesome, you could try blending in a little color yourself. Of course after you buy the white and color to mix in, it may have been cheaper to buy the mixed stuff. Besides, mixing the wrong 2 latexes together could be a disaster.