What colour do you get by mixing blue and yellow?

Mixing paint is subtractive: each pigment absorbs part of the spectrum and reflects the rest, so combining two pigments leaves only the wavelengths that neither one absorbed. A blue pigment reflects blue and a fair amount of green while soaking up red and orange; a yellow pigment reflects yellow and green while soaking up blue and violet. The only band that survives both filters is green, which is why the mixture darkens towards green rather than brightening. This is also why paint mixtures always get duller as you add more colours, and why enough of them together produce mud rather than white. Light behaves in the opposite way. Screens add wavelengths rather than remove them, so a blue beam and a yellow beam overlapping on a white wall give something close to white, not green. Anyone who has been surprised that stage lighting does not behave like a paint box has met exactly this difference.

Printers use cyan, magenta and yellow rather than blue, red and yellow, because those three subtract more cleanly and reproduce a wider range of colours.