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Light Intensity vs Spectrum

I've made a change to lighting recently and don't quite understand the

I have a combo metal halide hood over my 90 gallon tank. It has two 175w 5500K
metal halide bulbs and had two 40w fluorescent cool white bulbs. Last week I
swapped the cool white bulbs for some used Triton bulbs I got from a friend. 

The broad-leaf Ludwigia sp., Rotala macrandra and R. indica had up until this
time been mostly green with just a hint of red at the top. After a week with
the Triton bulbs, all three plants are now showing a much more intense red
(Ludwigia) and orange-red (Rotala) color in the new growth. I must say, the
new color is immensely attractive. 

I thought it might just be more intensity with the Tritons (they are supposed
to be much brighter than cool whites).  Using my luxmeter, I measured 15785 at
the water surface just above the R. macrandra with the Triton and metal halide
bulbs. I put the old cool whites back for awhile and measured 14125 lux at the
same spot. This doesn't seem like enough of a difference in intensity (about
10%) to make much difference unless the cool whites were "just on the edge"
(didn't produce quite enough light with the halides for full on growth). 

I've seen people say that the extra initial lumens from new bulbs can make
quite a difference and that for this reason you should change them every six
months even though they will still light for years. 

Does it make sense that different spectrums of light can dramatically affect
the color of plants? Tritons are a lot more red than cool whites. Maybe the
extra color is used by pigments other than chlorophyll?   

Forrest King
Checking  Mr. Booth's WebPage for things I missed the first time