That's the problem. Unless you have a gazillion watts of light, most stems color up only as they reach the top of the water. Keeping it trimmed will keep it green.
In the future, leave the stems in the back to grow to full height. When they start bending around as a result of being taller than the tank, trim them taking about 1/4 to 1/3 off the top and planting these (w/ red tops) in front of the taller stems. Eventually, you may well have a gradation of red tipped leaves from short to tall. Also - keep other plants from shading them.
Iron level management is not generally a good way to manage color.
well, I agree with you sqawkbert, except for the fact that it was all read as a bush then went through a growth spurt and I had to trim it back and it has never regained the red color it had as a bush prior to the trimming.