Sounds like you did a really cool experiment. Red, green, and brown algae have different types of pigments which give them their color. (Brown algae gets its color from the xanthophylls pigment fucoxanthin, red algae get their color from phycoerythrin, green is from chlorophyll.) These pigments have a certain chemical structure that allows them to absorb light. I'd imagine that different chemical structures might allow them to absorb light differently. Why would algae want to absorb light differently? Light is essential for algae to grow. And if there are lots of different species of algae in the same place they will compete for the same amount of light (for example, like five kids fighting over one cookie.) The cool thing about light is that it has many different wavelengths. So if one type of algae can absorb at one wavelength of light and another type of algae can absorb at a different wavelength of light, they no longer have to compete against each other and both can grow well. I hope this answers your question.

It is true that different types of algae absorb different wavelengths of light. Actually, it's not the algae, but the pigments they contain. Different types of algae have different pigments to maximize on the amount the light they can attract to their chloroplasts, where they perform photosynthesis. Usually the pigment variation is related to the habitat that the algae live in.