Is camo a MUST HAVE? NO! Although it will increase your calling success! This doesn't mean you have to go out and spend a ton of money on camo, use what you have, it will work just fine. You can see on my home page I'm wearing several different kinds of camo and I was still able to call in coyotes. It is more important to sit in the shade and have the sun at your back. Keep your movements to a minimum and don't sit yourself on the skyline.
Often guys wonder what rifle they should use to hunt coyotes, don't over complicate this, use what you have. If all you have is big caliber rifles, use them. I currently use a 223 and a 22-250, love them both. The 22-250 does get a little pricey to shoot, but not to bad when you reload. Dont be afraid to ask around and see what other rifles guys are using, you will soon find what works for you.