There could be a couple different things going on, but it's tough to say without knowing more. The light you see from a heterogeneous mixture is scattered light. In the case of a green laser pointer, your eye is more sensitive to green light, and the laser may be more powerful, so you may see more scattered light than with the red laser, even if everything else is equal. However, the green laser light is shorter wavelength, so it will likely interact (that is, be scattered) more strongly with smaller molecules and particles, also causing you to see more scattered light. (On this note, the sky is blue because the shorter wavelengths of light--like blue--are scattered more strongly than longer wavelengths--like red. Because we see the scattered light, the sky appears blue.)

All right, I'm not totally sure what the physics is behind the use of a laser to test for heterogeneity, but I'm guessing that it has to do with the fact that different substances have different indices of refraction, which spreads out the laser's beam. If so, then I also know that a substance's index of refraction depends on the frequency of the light being shone through it, in which case different materials might not scatter the light from a red laser if their indices of refraction are the same at that wavelength, but would scatter green light because their indices of refraction are different in that part of the spectrum. My guess is that's what's going on.