Textures that have been edited in a photo editor to be grayscale (turning down saturation to 0) seem to turn green and form weird shapes (in some cases, become small and duplicate themselves so the texture is double) if they're intended for items, and turn purple and green with black backgrounds for blocks (and block-breaking textures). The issue is easily fixed by going back to the photo editor and tinting the image even the smallest amount.

One other thing I noticed specifically with Elytra was when I made a texture for the Elytra, and then grayscaled the texture and put it on the Broken Elytra. Once I held the Broken Elytra, the normal Elytra was forever grayscale until a relog. Not sure if that's because they're the same texture, or because Elytra have some strange bug relating to Broken Elytra where they render the same texture.

Steps to reproduce:
1. Create or find a texture
2. Send to a photo editor and look for "adjust colors" or "saturation"
3. Turn down saturation to 0
4. Use a texture pack containing the texture
5. Obtain one of the items in-game

It appears this bug may affect textures made using a grayscale palette, but I've gotten inconsistent results (probably due to the texture makers I use). I do know that some textures that are grayscale that I've found online glitch out, and I suspect those were made manually and without a grayscale editor