Why are the lines on many tzitzis on taleisim begadim?
Peri Megadim (Eshel Avraham 9:6) writes that the custom was to have blue stripes on the talis, which came to remind one of the techeiles. The custom, however, is to have black stripes on the talis. This is possibly because the ink that the techeiles was made from was black (See Rambam, Tzitzis 2:8 and 2:2). Alternatively, it is so that people won’t think that this is actual techeiles.
Peri Megadim explains that although a talis should be the same color as the strings, the stripes do not constitute a problem, because we follow the principle color of the garment.