Hm I'm a little skeptical, can you describe the math behind this? It doesn't look like the light is adding any color to the surface, only replacing it.

I think it's fine. It's common for diffuse color to be calculated with

lightColor * surfaceColor * diffuse * attenuation

Where lightColor and surfaceColor are unit vectors and diffuse and attenuation are scalars.

He does lightColor * surfaceColor with lightWith, the diffuse * attenuation portion is taken care of with lighten.

I haven't tested anything, just going off of some graphics shaders I have.

Note: diffuse in the example is brightness in this code and I think you are using attenuation as colorDifference.

Awesome, thanks for the explanation.

Landed in d3b6336.

I went ahead and made it so Color.prototype.lighten takes an optional second parameter, lightColor, instead of having two closely-named methods.

I'm okay with merging this because the default behavior is still the same. Thanks!

Cool! I couldn't find an explanation online of how colored lighting works, so I based my solution on the thought that the visible color of an object is just the color of the light it reflects, and therefore a light that provides less of a certain color gives less to reflect.

More formal explanation can be found in Lambertian reflectance and looking up flat shading.

There is also a whole lot of explanation from some graphics tutorials, like this.