Dark barley water

It is unlikely that pure barley, however strong the malting, will produce such a dark color. I think it is safe to assume that it is colored.

In Germany, this (or a very similar) beverage is called Malzbier (See the English Wikipedia article on Malzbier) It is an alcohol free drink made with malted barley, and commonly colored with the food coloring E 150 (Look at the site of this popular drink, third FAQ item from the end). This is a coloring created by letting different types of sugar undergo a Maillard reaction, and can be considered a type of caramel. So it can be used in a malt drink in relatively high amounts without introducing an unusual taste or presenting a health risk.

If you have seen lighter barley water until now, this is probably because different producers use different amounts of coloring. It is probably a regional thing, the drink in the picture is produced in Spain, maybe they like their malted drinks darker.

It could be that the producer uses toasted or roasted barley as part of their process, which would impart the darker colors that you would normally see in various styles of beer. That would also give it different flavors. Toasted malts could provide nutty, toffee, caramel, or raisin flavors. Roasted malts would give it more coffee or chocolate flavors.