Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
Why is the sky blue, and sunsets red? (for a curious adult who has never studied physics). Cover, in this order: What is white light made of?; What does air do to a beam of light?; Why does blue light scatter more than red?; So why isn't the sky violet?; Why are sunsets red?; Why are clouds white? — page 1: What White Light Is
Sunlight looks plain white. But white isn't one thing. It's a whole bundle of colors, all traveling together in a single beam.
Colored flashlights overlapping on a wall add their light together; where three meet, the spot turns white. Paint works oppositely: it absorbs light, so mixing paints goes muddy.
A prism bends light, but each color bends by a different amount. Violet bends most and red least, so the white beam fans out into a rainbow.
The fan runs red, orange, yellow, green, blue, violet. The prism makes no color; it only sorts the colors already hidden in sunlight.
So sunlight is a team of colors, not one color. Next, we'll see what happens when that team runs into air, because air treats each color differently.