The riddle
What Is Blue and Smells Like Red Paint?
A riddle with no trick
Most riddles hide a surprise. This one hides nothing. The listener hears blue and red together and starts looking for something purple, or something that belongs to a painter. The answer is blue paint, because blue paint smells like red paint. It is an anti-riddle: the humor comes from the listener expecting a clever answer and the reply refusing to give one.
Where the color comes from
Wikipedia says the parts of a paint are a binder, which forms the film, pigment particles, which usually supply the color, a solvent that keeps the paint fluid and evaporates, and small additives. The solvent is usually water or an organic solvent. Dyes can also color paint, but they dissolve instead of sitting as particles. The color is the pigment's job, and the evaporating liquid is a different ingredient.
Where the smell comes from
The U.S. Environmental Protection Agency notes that paints, varnishes and wax contain organic solvents and can release organic compounds while they are in use and, to a degree, in storage. Those compounds come from the solvents and not from the pigment, so two cans of the same kind of paint tend to smell alike whatever the color on the label.
The mirror and your own
Searchers also type the reverse, "what is red and smells like blue paint", and the answer is red paint. Making a new one takes only a color, an object and a second color: what is yellow and smells like orange paint? Yellow paint. A neighboring anti-riddle is what is orange and sounds like a parrot.