Some commercial brands are light-coloured, some such as the Torani brand are dark. Commercial brands made in the West are often white grape juice concentrate with a Brix (sweetness point) of around 68. A few California grape growers are making it (as of 2007.)
Some Western make-your-own recipes have you simmer the grapes until the skins burst, strain, then add cornstarch, apple cider, and sugar (sic.)
You can make your own from 2 x 12 oz (350 ml) cans of frozen concentrated white grape juice.
Empty the contents into a frying pan (to give it a wide surface for evaporation.) Cook on high heat until it boils and foams (about 10 minutes), then reduce heat to medium, and cook until the volume has reduced by half (about 20 additional minutes.) Let cool, put in sealed container, and refrigerate. Bring to room temperature before using. You can use white grape juice, or red grape juice. Yield 1 1/2 cups (12 oz / ml).
The Ancient Romans and Greeks made grape syrups such as Defritum and Sapa (Sapa being the more concentrated one, according to Pliny.) The Hebrews also made grape syrup; they called it “yain.”
In Turkey, Grape Syrup is called “pekmez.” The Greeks call it “petimezi.”