Many of you may have heard that supercritical CO2 (as in at just the right temperature and pressure to be in between gas and liquid) is a possible alternative solvent for life, like water is for us. But this brings up so many questions. Let's focus on this one for now: What wavelengths of light would go through such an atmosphere? This is important because it affects the color of the autotrophs. Let's assume the star is like our sun just for simplicity.
Such planet would be similar to Venus. And question is more about reflection of light and rayleigh scattering. Your thick atmosphere will reflect a lot of light and scatter what is left, as rayleigh scattering is dependent on wavelength of light: shorter wavelengths blue/green won't make it to the surface, most light will be orange and red wavelenghts. Sunset would be good reference. Photos of Venus:
Take a look at this picture - I haven't found something similar in English, but "Kohlenstoffdioxid" is CO2, and at the bottom there is the wavelength, which is blocked by the different gases. Also take a look at "atmospheric windows".