You can usually spot hydrogen flasks on the cylinder valve cap, since you have to unscrew it clockwise.

ITYM the CGA connection. The cylinder valve connections for flammable, toxic and corrosive products typically have a maie connection with a left-handed thread (e.g., CGA 350).

Oxidizers typically have a female left-handed thread (e.g., CGA 540).

Inerts have a male right handed thread. (E.g., CGA 580.)

I can't remember what air has, of the top of my head.

You can identify the "handedness" of a connection by looking at the nut on the nipple. If there are notches cut into the corners of the nut, it's a let-handed thread. (Old propane tank connections are a good example.)

"Right-handed" is the threading that people are most accustomed to. To figure out which way to turn a right-handed threaded nut or whatever, curl the fingers of your right hand and stick out your thumb. If you turn the nut in the direction your fingers are pointing, the the will go the direction your thumb is pointing. Ditto for left-handed threads and your left hand.

Cylinder "caps" are the protective caps that cover, or at least surround, the cylinder valves. They're all right-handed threads.

And, as I mentioned above, never rely on the color of a cylinder to indicate its contents. Always check the decal (label).

The special mixtures I have ordered is also red and they contains hydrocarbon gas.

If you're ordering a true specialty gas, then the cylinder is typically brand-colored. Meaning, all the specialty gases mixes from a given vendor tend to be the same color. E.g., all the spec. gas blends from Praxair (PDI, technically) are cranberry. (Except for aluminum or stainless steel cylinders, which often aren't painted.)