ELI5: Watercolor vs Gouache: What's the Difference?

Watercolor is a see-through paint that lets the paper shine from underneath, while gouache is a solid paint that completely covers up whatever is behind it.

Think about drinks you have at home:

• Watercolor is like apple juice. When you look through a glass of apple juice, you can still see your fingers on the other side. If you paint a red line with watercolor and then try to paint a white line on top, you will still see the red line because the paint is transparent.
• Gouache is like chocolate milk. If you look through a glass of chocolate milk, you cannot see anything through it at all. It is thick and opaque. If you paint a red line and let it dry, you can paint a bright white gouache line right on top, and the red will completely disappear.

• Stained Glass vs. a Brick Wall: Watercolor works like colorful stained glass on a sunny day—it needs the bright white paper behind it to glow. Gouache works like a painted brick wall—it creates a solid, flat block of color that hides everything behind it.
• Light to Dark vs. Any Order: Because watercolor is see-through, you have to be very careful. You must paint the lightest colors first, like yellow sunshine, before you paint the dark colors, like a brown tree trunk. With gouache, it does not matter! You can paint a dark blue night sky first, and then stamp bright yellow stars right on top because gouache is strong enough to hide the dark background.
• Fixing Mistakes: If you spill grape juice on a white shirt, it is very hard to make that spot white again; that is like making a mistake with watercolor. But if you get a smudge on a wall, you can just paint right over it with a fresh coat of solid paint; that is how gouache works.