Is there a light sensor that varies output with wavelength?
Basically all bandgap devices (read: semiconductor devices) are specifically sensitive to the wavelength for which photons have exactly the energy needed bring an electron from lower to higher energy band. So, for example, LEDs (light emitting diodes) work pretty well as selective photodetectors.

So, a blue LED will be significantly more sensitive to blue light than to red light.

Technically, it's much harder to do the opposite of what you want: have an output that depends only on intensity, not on wavelength of the incident light.

 

This is really simple to test: Get a monocolor LED. Take Blue.

Now, there's different ways to use an LED as photodiode, but I think the easiest for you will be to use the LED, and connect it in reverse to a (stable) voltage source (e.g. 4.5V coming from three AA batteries). Use a sensitive multimeter in series to measure how much current passes through the LED in reverse direction.

First, cover your LED in darkness, measure current, then expose it to bright blue light – another LED (operated in forward to actually emit light) of the same kind, and compare the current readings. Then, compare current readings when subjected to other colors of light – from a RED LED, for example.

If your ammeter isn't sensitive enough for µA to mA, you might want to feed the current to the base of a transistor pair in Darlington configuration.