The interesting thing of the Foucault test is that it is measuring the radius of a particular zone. This means that the wavelength of the light used doesn't matter. However, the color of the light does affect the shading of the mirror in that the blue end does give a sharper, higher contrast shadow. I do believe (please don't quote me on this though) that the wavelength used as the standard is given as the yellow-green area around 550nm. I might note that I have never seen "colored" shadows when I was working with a incadesent bulb for light and this reinforces the idea that the test is essentially "colorfree". The difference in the shadow if the colors used in the test were a deep red and blue should be less than 2:1 and that's not a very strong difference considering the view. The focal points will be the same for each of the colors so you won't be seeing any prism effects with the colors. Using a security camera (put the camera so that the lens is half behind the knife-edge and focus the camera on the mirror) has shown no real difference in the contrast, probably because the increased sensitivity to the red counterbalances the decreased shadow of the test and any color of led is probably quite useful in testing with a camera. Hope this answers your questions.