The senescence-rejuvenation cycle is increased by high temperature resulting in the change in colour. Please check the following links for more details.

http://www.fao.org/ag/againfo/resources/documents/DW/Dw2.htm

https://www.academia.edu/6741780/The_Azolla_Cooking_and_Cultivation_Project

Here i attach the link to a similar study.  hopefully this can help.

http://m.aob.oxfordjournals.org/content/56/5/701.short

Bijaya:

We do not exactly know as to what functional advantage Azolla pinnata or other species could have in changing color from dark green to hues of Brown-Red under intense solar radiation and high tolerable temperatures: Kindly have a look at this link:

http://theazollafoundation.org/growing-azolla/

Best

Syed

The red colour of Azolla pinnata is due to development of anthocyanin pigment under phosphorus deficient condition.

Dear Bijaya

Arvind is correct I have found in the field that at high temperature under phosphorus deficient places the Azolla plants starts to develops Anthocyanin pigment and it turns reddish green in colour.  but I am unable to explain the change of colour under high temperature from green to brown. 

Sunit

The synthesis of anthocyanins is induced by UV-B (Warren et al., 2003), nutrient deficiency (Pinto et al., 1999), low temperatures (Rabino and Mancinelli, 1986), water deficit (Nogue´s et al., 1998), and heavy metal stress (Ling-Peng et al., 2006). The content of the most common plant pigments—chlorophylls, carotenoids, and anthocyanins—as well as their relative proportion determines both the color and appearance of plants (Abbott, 1999).

A copy and paste from a paper by TAYLAN KOSESAKAL

You can request from the article from him.

Article Effects of Seasonal Changes on Pigment Composition of Azolla...

MRRK:

The question asked is Why the color changes and not how, which is fairly well explained. What functional advantage / necessity Azolla plant will have in changing colors?

Best

Syed

Under intense sunlight, the green change to red due to degradation of chlorophyll. This in turn change the colour of Azolla into red. In a way this reduce photosynthetic activity. Reduce photosynthetic activity means reduce growth.

So Syed Abbas Jafar , we all know Azolla proliferate very quickly. Do you think it is a natural response to prevent extreme overgrowth during summer/etc?

The change in color is due to anthocyanin pigment production.