Need advice re: Guzmania and Vriesea

[e.t. and dct]

I have a Guzmania "Vella" and a Vriesea "Isabel".  Both plants appear to be
growing well; the leaves are in great condition, and are a beautiful green.
But...the flowers seem to have lost their color.  The Guzmania has turned a
yellowish green in color instead of the bright red it is supposed to be.
The Vriesea flower is not bright yellow and red now; it's also a yellowish
green.

They are both indoors, but they are under a plant light.  Can anyone tell me
what has happened and how to resolve this situation?  Thank you!  e.t.