I just went through more of this than I can to ever do again so I'll pass on my lessons I've learned the hard way over the past year. Here's the pile of butyl tape that I have peeled off my camper during the roof overhaul process. Some of it is old stuff and some of it is new excess trimmings.
As you found out, use a putty knife to remove the bulk of the butyl tape. What type of knife you use will depend on what surface it is stuck to and what the condition of the tape is. Metal or plastic. Plastic dinner knifes work pretty good in some cases too. For me, I used a metal flat blade putty knife to gently scrape the flat surface of the camper to get most of it off. Since the skin of my camper was Filon (gel coated fiberglass sheet) I was able to use Acetone to get rid of the residue. If it's in a crack or tight spot then I would dip a plastic cleaning brush in acetone and scrub the surface to get clean and then wipe with acetone rags. If the tape is on painted metal then I would advise against anything acetone unless you plan to repaint.
Warm butyl tape is sticky butyl tape. Getting it off, it's better cool. Putting new parts on, it's better warm (stickier and squeezes excess out). If you don't squeeze out the excess then when it does warm up and you put a load on one of those jack brackets then it will come out thus loosening the bracket bolts.
After putting everything back together and cutting the excess off (putty knife on edge) I caulked the borders to keep it from coming out and make a cleaner seam. All the seams that had unsealed butyl had turned nasty over the years of collecting dirt.
They make a gray and a white in different thicknesses, shapes, and widths. Not sure color matters in most cases cause not much of it shows.
