This is the 16808th most frequent Russian word.
смешать
To mix.
The verb "смешал" (from "смешать") is used to describe the action of combining different elements to form a new mixture, in this case, paints.
Я смешал краски, чтобы получить новый цвет.
I mixed the paints to create a new color.
The verb "смешивай" (imperative form of "смешать") in this sentence warns against combining items from separate groups, emphasizing organization or separation.
Не смешивай фотографии из двух разных альбомов!
Do not mix photos from two different albums!
Here, "смешались" (past form, reflexive of "смешать") conveys the meaning of merging or blending into a group, losing individual distinction.