Why does my rainwater turn yellow if I increase the pH to 7?

My rainwater is colourless when collected, but if I add a small amount of sodium hydroxide to increase the pH to about 7, after a few days it goes yellow.
The colour change happens more quickly if I increase the temperature.
This change to yellow is accompanied by the disappearance of any dinoflagellates or cryptophytes which were living in the water (determined by 300X microscope.)
The rainwater appears to contain a significant amount of dissolved organic carbon.
The rainwater is collected from the PVC roof of my shed in a covered plastic container which I have recently cleaned.
The pH of the rainwater is between 5.4 and 6 when collected (the best I can do with bromothymol blue indicator solution and narrow-range indicator paper.)
I am located in the south-west of England about one mile from the sea.


