Only Retzuos "Avodas Yad" (hand-made) are recommended. Retzuos which are machine-made are much cheaper, but are halachachly not preferable. Many prefer Retzuos which are black on both sides. The main reason: Since it is important that Retzuos always maintain its black color, these will never wear off, since they're soaked in black paint through and through. But besides, some Rishonim are of the opinion that Retzuos should in any case be black on both sides.