Rosehip Mole asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
Blue ball
Here's a simple blue ball. Sunlight hits it, and sunlight is actually made of every color mixed together.
The paint on the ball's surface is a picky filter — it soaks up red, orange, yellow, and green light as heat.
But blue light bounces right off the surface, unabsorbed — that reflected blue light is what shoots into your eye.
Your eye's cone cells fire strongest for that blue wavelength, and your brain reads that signal and simply calls it: blue.
The ball isn't full of blue — it's just the one color it refuses to keep, so that's the one you see.