I recently completed the Hydro-Turf installation on my 180 Challenger. I ended up choosing the dark grey after getting a sample of the light grey and deciding it looked to "green". It probably would be OK if I hadn't compared it with the carpet. The light grey is closer in lightness to the existing carpet, but I think the dark grey works. I would get a sample of the blue, light and dark grey and then decide. People have said that the brighter colors fade, so beware of that. I'm sure it won't happen in a couple of months.
Anyway, as far as installation goes… I started by removing the carpet. Then I thoroughly cleaned the floor. Next, I removed the snap studs for the carpet using a common phillips screwdriver. They just screw out like a regular sheet metal screws. In my boat, there were 4 studs that weren't going to be covered by the Hydro-Turf, so I left them in. I felt it was better than having a hole. The two that weren't going to be covered are the two in the front of the boat, just past the floor storage and the two at aft-most part of the boat near the small cover/hatch (the innermost snaps, not the ones near the sides of the boat).
After removing the snaps, I put a dab of clear silicon caulk over the hole and used a plastic putty knife to fill the hole and scrape off the excess. I then laid out the pieces where they go to make sure I put the right ones in the correct spot. (The small pad that covers the step you use to enter the boat is almost the same size and shape as the foot rests for the captains chairs. You have to look carefully to make sure you use the right piece.)
I then worked from the front of the boat back. I bought the kit that has the 3M pressure-sensitive adhesive on advice of basically everyone on this forum that has Hydro-Turf. It's great, because you can lay down a piece gently, and if it isn't lined up quite right, you can pick it up and try again. And again.
I found that the textured parts of the floor make good guides for where to install the pieces. Once I was satisfied with the location, I smoothed out the piece, first with my hands, then I used a laminate roller (available at Home Depot or Lowe's) to apply heavy pressure for a good stick, working from the inside toward the edges to avoid trapping any air bubbles. A quick test trying to peel up the edges let me know if I got it down well.
If you decide to go for just the swim platform and you choose dark grey, let me know--I happen to have an extra. If you want to see pictures, I can take some and post them.