Why not have tigers for the knife? I would love to have a different coloured knife handle and its possible to get head shots with the knife.
The knife should definitely have its own challenges like all the other weapons in the game. 80 - 90% of my kills are via knife and i would like to be rewarded for it. Some sort of Knife stats would be nice as well.
Knife camo and Grenade camo are more useless than Gun camo (somehow) and could only be seen sparingly.
Although it would be cool to see more challenges than just "Get 3 Knife kills in a row" and "Get 15 Knife Kills" I don't think camo makes sense, because even though you "can" headshot people with the knife, it gives no advantage over knifing them in the calf.
The different tigers you get for the other guns are not really effective in camoflaging (sp?) your gun, what use is red or blue tiger for hiding your gun? I would say none really, its more for aesthetic purposes to make your gun look cooler and that should be available for the knife.
I didnt know there was a 15 kill knife challenge? I have a 16 kill knife streak and didnt get anything for it but that was including a airstrike and chopper so maybe that didnt count?
The 15 kill knife challenge is just for getting 15 knife kills overall. Whether it took you 1 game or 20 it will still give you the challenge when you have completed the 15th kill. You probably already have it completed, but don't remember it.