I saw these Tzitzit on ebay, I was wondering what was halachicly wrong with them? (at least according to the Rambam)
Hilchot Tzitzit Chapter 2 Halacha 8
When a garment is entirely red, green, or any other color [besides white], its white strands should be made from the same color as the garment itself. If it is green, they should be green. If it is red, they should be red.
Should the garment itself be techelet, its white strands should be made from any color other than black,
for it resembles techelet. He should wind one strand of techelet around all the strands, as one does with other tzitzit that are not colored.
ח טְלִית שְׁהִיא כֻּלָּהּ אֲדֻמָּה, אוֹ יְרֻקָּה, אוֹ מִשְּׁאָר צִבְעוֹנִים–עוֹשֶׂה חוּטֵי לָבָן שֶׁלָּהּ, כְּעֵין צְבָעָהּ: אִם יְרֻקָּה, יְרֻקִּין; וְאִם אֲדֻמָּה, אֲדֻמִּין. הָיְתָה כֻּלָּהּ תְּכֵלֶת–עוֹשֶׂה לָבָן שֶׁלָּהּ מִשְּׁאָר צִבְעוֹנִים, חוּץ מִן הַשָּׁחוֹר, מִפְּנֵי שְׁהוּא נִרְאֶה כִּתְכֵלֶת; וְכוֹרֵךְ עַל הַכֹּל חוּט אֶחָד תְּכֵלֶת, כְּדֶרֶךְ שֶׁעוֹשֶׂה בִּשְׁאָר צִיצִיּוֹת שְׁאֵינָן צְבוּעִין.