Dialed.gg is a simple browser game that shows you a color, hides it, asks you to recreate it from memory, and scores you on how well you did. They recently added similar tests for sound, time, and shapes, too. I did OK on the color and time recall test, and not as well on the sounds and shapes. The Internet Archive has a full scan of How It Works... The Computer, a 1971 Ladybird Books children’s guide by David Carey, illustrated by B.H. Robinson.