Most of the loss of light being transmitted through individual cells is not absorbed, rather it is scattered (redirected from its original direction to a new one without loss of energy). There is actually not very much in cells that can absorb visible light, and virtually nothing at all that can absorb many red or near-infrared wavelengths.
Scattering does depend on wavelength, but the relationship is very complex. For scattering from small cellular components (mitochondria for example), Rayleigh scattering dominates, which will decrease with the 4th power of wavelength, so that green light will scatter much more strongly than red. For larger components that are comparable or larger than a wavelength (nuclei or whole cells), scattering will increase much more slowly. The relationship between the direction of scattering and the size/wavelength is even more complex, but is very useful in applications like label-free cytometry where cells can be sorted based on scattered light.
600-800 nm light is a reasonable choice because it will not damage the cells unless the power level is very high, although using shorter wavelengths may give you more sensitive measurements.