Oh! Well, since I'm in no way the best at explaining things, I went ahead and found some guides I think are pretty great.
This one specifically will help with understanding the lock alpha feature. :D
This is a very good general Oekaki guide, and
this has all the answers to frequent Oekaki questions. ^^
As for who can edit, right now you have it posted in the Editable section (happens to the best of us), so anyone can go in and color it from your post. Usually colored editables go into the Colored-In Oekaki board. A mod would be glad to move it for you. Otherwise, there will be a box above your initial post, below your actual Oekaki, where you can type in the usernames of people you wish to grant access to "color in" your post. c: