You are seeing Green Spot Algae which usually shows up when phosphates are low. I've found that keeping phosphates at 3ppm will usually keep GSA from forming. You can also move it to a more shaded location which helps.
Non-green plants color up best in high light. While iron can help with coloring them up running a low nitrate (10ppm), higher phosphate tank (3ppm) will greatly improve color. Plants need phosphates for the chemical change they undergo when coloring up. But overall the best colors are achieved from higher Par levels. While I get some decent color in my medium light tanks I get the best and most intense reds, pinks, yellow, and orange from high levels of light.