If you plan on buying buy supplies from Springfield Leather, I would suggest joining their wholesale club. The set you bought is $44 less if your a member of the club.
I've done what you want to do dozens of times. What I usually do is dye the whole piece brown, wait for it to dry completely, then carefully dye the part you want black. I've never had any problems doing it that way.
You could try and to put latex tubing over your tools to make the diameter larger.
http://www.lowes.com/pd_35791-104-RLGE_0__?productId=3419514&Ntt=latex+tubing&pl=1¤tURL=%3FNtt%3Dlatex%2Btubing&facetInfo=
I'm not good enough yet to do Saddle Bags from just a pattern yet, and I don't have the correct leather weight, so I was wondering if anyone knows where I can get a kit for an old west Saddle Bag?