Sometimes plants will increase their red/orange pigment as a result of high pH. This may be the cause. Other times it is a result of too much light. Are they otherwise healthy? If so, I wouldn't worry about it too much.
The picture below is taken when the water is 0 gH 0 no3 0 po4. Bascially when I did not dose anything in the water and the fishload was light.
Ca is what effect the size of val. the more Ca you have the bigger it get. This is the info I got from tropica a while back.
As for the reddish coloring, I am not sure why it turn red. the pH had always been <7.
When the PC bulbs on my tank were <1/2 year old, I notice the reddish growth on the vals near the substrate. The vals only show the red coloring when they are growing across the surface now. The bulbs are close to 1 year old.