There not available commercially (at least I haven't found them), you have to make your own. Get yourself some oil based paint pens like the ones below, they make fine point ones but you'll want the larger tip ones. Amazon has sets of them in different colors too, just be sure to get the oil based ones.
Wipe the hose clean with lacquer thinner, chemtool or suitable cleaner to remove any oils or lubricants.
The trick is how you use the pen, clamp one end of the hose in a vice and pull it straight as you slide the pen along it's length at roughly 90 degrees from the hose. Don't use the end of the tip, the line will go all over the place. Use the side of the tip and drag it along the hose as you pull the hose straight, keep the pen at about the same angle with light pressure as you slide it along. If the ink starts to get a little dry rotate the pen in place as you pull it along. Set the piece aside to let it dry, it will give drying times on the package.
A little practice and you'll be a pro at making a nice even line all the way down the hose in whatever color you choose.
And don't worry about line thickness, the different hose diameters will have different line sizes because the contact patch of the pen is different. Smaller hose, smaller contact patch, smaller line, it works pretty slick.