Another way to create a clipping mask is by holding down alt, and then click in between the layers you want to clip, your cursor shall change. If that doesn't work, your version of photoshop may not use clipping masks.
You mean when you change the color temporarily or when you use the gradient? You can skip the step of when you color over the white, it shouldn't effect anything.
Hm :/ I couldn't tell you what went wrong, but maybe your lines weren't closed, you forgot to make a clipping mask on one of the layers, you forgot to hit select inverse at one time, or something else.
Idk, maybe make just a small quick picture just to test out the method, and practice a bit until you get the hang of it c: