It should work for the Sam--->Pippin cloak... The Frodo --->Merry is the one that will be challenging. Getting green to an even pearl grey will be a task. Thats going to be my weekend project.
Honestly, Pippin coat doesn't worry me much at all. At the very worst I'll have to use a bit of bleach before I apply the Teal dye... We'll see. I am the group dyeing guinea pig after all.
I think it'll be best to try and remove the color from an item before dyeing like you did with the vest, even if going darker, to get a purer color rather than it mixed. Starting with a blank canvas. But, of course, it's up to you. I suppose there are risks either way.
What pic is that from? Is it some fan made one? The colors look completely different, esp the fringe, from every other reference photo of the costume I've seen.