i once asked an old carpenter what the difference was between an apprentice and a master carpenter. He laughed and said the master knows how to hide his screwups.
the way i see it is you have three choices .
1. an apprentice = keep trying to dye the edges perfectly and throw all your screwups in a bin and lose money.
2. Journeyman =Dye the whole thing and sell it for the same price and hope the dye doesn't bleed.
3. Master= cover it with a liner and sell it as the high QUALITY leather work you want to sell, and you can do it for more money.
Just my opinion but you commented on another post that you wanted to sell high quality leather work, well an unlined collar isn't high quality work.
How ever if you are intent on selling unlined collars then
I use an edge paint and apply it with the wooden end of a small brush, simply dip about an inch of it in the paint wipe the extra drip off on the container and run it along the edge using the side of the handle