Anne, I do something similar, but instead of painting the bottles, I just put used coffee grounds in the water. Makes the water a nice dark brown.
We buy 2 1/2gal. jugs of water for our yearly hurricane supplies, and after we use the water up, I refill them for heat sinks, too. C/w coffee grounds. The bigger jugs stay warm longer.
I had a friend in Salt Lake City who heated his greenhouse with those plastic storage tubs from Wal-Mart, filled with water. They not only warmed up in the sun, they helped humidify the air, too. If it got really cold and through the short days in winter, he would sometimes use a waterbed heater to keep the water a little warmer. They come with a thermostat and everything. Very energy efficient.
