Why do the Ashkenazi Tallitot have stripes on them?

Shalom, Thank you for your question. The stripes on the tallit are a custom, and there are those who use totally white, or even other colors - in essence it really doesn’t matter. However, you are correct that many people have black or blue stripes on their tallit. What is the source for this? I saw three reasons quoted – Some people suggest that the stripes are in memory of the blue thread (techellet) that use to be part of the tzitzit fringe (and which some people say they have re-identified and use today) [Pri Megadim, Orach Chaim siman 9, 6]. Others explain that according to the Zohar the black or blue stripes represents the attribute of G-d's might (gevurah) and the white tallit represents G-d's mercy (chesed) [Zohar Vol. 3 p. 227a]. Lastly, some explain that the stripes are used to be able to see if the sun has risen enough to allow the recital of the morning Shema – which, according to the mishna may be recited only when it is light enough for one to distinguish between white and blue (techelet). Originally the blue thread in the tzitzit was used to check this timing, but now we use the stripe on the tallit itself. Blessings.