I'm a sea kayaker and am looking at flags to attract attention if I can't roll up after a capsize.

What color is best in hazy conditions?

In my experience windsurfing, it seems that white is the most visible. Sailors have brightly colors sails so they don't run into each other, and lime green and day-glo orange are common. White stands out better in most circumstances.

Dunno if this might prove handy:
RescueStreamer Link

Harley

This is just a guess, but I'd have to say blaze orange... bright, highly reflective, and nothing in nature that I can think of is that color. For real eye catching ability, you might want to stripe it with neon green, both are danger/warning colors... it might be ugly as a mud fence, but anybody that glances at it WON'T miss it.

Troy

I have a pretty compact kit from Orion Safety in each of our Kayak's that has the following items for signalling:
1 orange plastic streamer
3 handheld red meteor flares
1 handheld orange smoke signal
1 flourescent green dye marker
1 whistle
1 signal mirror
1 waterproof flashlight

If you get in trouble, putting a flag on the end of your paddle and waiving it will get the attention of anyone close by, but, you need some other options if you're not near someone and want to get noticed. Remember that you want signals that can be readily seen from land and water (at your level basically) and also from aircraft above. It's a little redundant but we carry a whistle, signal mirror, and lightstick attached to our life jackets (in a front pocket on a lanyard). Having a strobe, or even just a lightstick, will significantly improve the ability to be found at night.