Doug, I like the Darker Blues (not real dark though), IMO, they look 'best' as the water and bright sun tends to wash the clour out a bit.
The missus just had our pool repainted a couple of years back and of course that came with the agonising 'what colour?' We decided on a darker blue (it was fairly faded before) and at first, we thought we'd made a mistake, but once it was filled, it looked 'just right'.
<edit: I recon the Diamond Blue Super Bright or Midnight Blue>
Nigel, mate, please, there is a standing offer on the table mate, you know that, all you have to do is say the word. It's not our fault that you choose to live in a "cold, dark, wet and did I say cold here" place. I live in a hot, bright, not so wet, but humid place and there are days that I don't like either. I guess while you guys spend a lot of cash keeping warm, likewise, we spend zillions on keeping cool. I hate the heat, and actually look forward to the winter
Doug, when it's done mate, I'll have to pop over for a dip, just to make sure that it's done right.
Old Mokers never die, they just smell that way.
has the final vote + veto). 
