Flexible Hose

One reason is to make it easy to identify different hoses that otherwise look similar.

Is red positive and black negative?

Identification between two similar products so the guys pulling it don't send out the good stuff when the cheaper was ordered.

And, color doesn't necessarily have anything to do with cost.

The color differences are used to denote working and burst test pressures, material composition differences, design use of tubing etc...

http://www.jfoakes.com/Chemoak%20Hoses.htm#Construction/Mining

Is one color for COKE and another color is for DIET Coke, maybe ?

Red color specify - Fire fighting Hose

Balck - Normal Hose

Red hoses are faster.

All joking aside, there is a serious reason for the color difference. The red hoses are rated at a different temperature and pressure than the black hoses. To apply live steam to a water hose spells disaster and injury to employees. Yes the red ones are cheaper, but is someone's life worth the difference. If you need the black one buy it and use it. If you don't then don't.

Colour aside, what does the writing stamped/printed on the two different hoses say?

Yes, the red one is for fire fighting system, remaining for other application. I think color not affect price of hose. But one vendor told me the red color is more expensive (so that it embarrass me)

Don't worry about being embarrassed.

Take your money elsewhere if the vendor was a smart butt.

....and tell him where he can stick his red hose too.