That's neat project, and a colorimeter will be a useful tool for you in measuring algae concentration. As you probably know already, cyanobacteria get their energy from photosynthesis. To capture light for this process, they use a special protein called phycocyanin. This protein is what gives cyanobacteria their distinctive bluish hue. Not coincidentally, phycocyanin absorbs orange and red light, particularly light with a wavelength of about 620 nanometers. Because cyanobacteria are full of phycocyanin, I think you should use a wavelength of 620 nanometers. If you can't be that specific, use a wavelength in the visible orange-red range between 580 and 630 nanometers. Reference: Glazer, A.N., 1989. Light guides. Directional energy transfer in a photosynthetic antenna. J. Biol. Chem. 264:1-4.