I found both dark and light rye bread at the store but it was the factory-made kind. I definitely made mistakes with both batches including adding way too much bread to the light kvass.the bottles need a day to carbonate then we shall see how well they turned out. Not that I have any reference for the flavor.
Kvass flavor is generally light and fresh, bread-like, a bit sour, can be lightly sweet, floral - a lot depends on ingredients and technology.
It has a softer aftertaste and is generally low on that smell after you drank it compared to beer.
Source: Russian, drank a lot of kvass