If you want pretty color options, go with Tandy Pro Waterstain. This is the only tandy coloring product I use and I have tried many others that are junk. Just make sure you rub it in well, buff, apply thin layer of finish, buff again. Or, use an airbrush (my preferred method). Also, don't finish with acrylic resolene (the two don't mix well). Instead, use the pro finish that Tandy sells. I can give you better instructions on using Tandy waterstains if you prefer the colors, as this is the primary product I've been using for about a year. I know Tandy catches a lot of well-deserved slack, but when applied correctly and with effort, the color holds fast to the dog collars I make.
If you just want browns and maybe a basic red, green and blue, use Fiebings dyes. The only reason I don't use fiebings more often is because of the color choices, but I absolutely love the Fiebings pro oil dyes. The only other Fiebings product I use is obviously the acrylic resolene and the black edge kote. I'm not a fiebing's expert by any means other than knowing the company has been around for a while and sells very nice products.
A big factor with how well dye applies evenly and holds up over time is the quality of leather you use, too. I was gobsmacked when I finally invested in a nice piece of Hermann Oak - a huge difference