when ur in game u can check the skin settings menu for the optifine cape button, this takes you to the cape customization website. at that point you just change the colors of everything on the cape (the top, bottom, and text) to the same color.
The best server hosting service in my experience is shockbyte.com. You have to pay for the servers, but everything is at a very good cost. You can get a small server just for you and your friends for just around $5 a month. If you refuse to use money, I recommend aternos.org. The servers obviously don't run super well since its free, but its still very good for what it is.
Illagers are a great idea! Unfortunately, while armor can be placed on illagers (or any mob for that matter) it does not display on them, therefore there's really no point in giving it to them :/