If you buy white, you should be able to make whatever colour you want with some acrylic colourant. I've done this myself with colourant from Polyvine. I've never tried tinting white - so I've no idea how much you'd need. To get a good textured finish, you'll need the special foam rollers from Blue Aran. They're not the same as the ones you can buy in DIY stores - the foam is much coarser. You can buy them from Ali Express but I haven't seen them anywhere else. If anyone knows another source, that would be useful.
If you can recess the input panel, you'll achieve a better seal - and it will also look better - but surface mounting works also. Don't for goodness sakes mess about with sealant. Use adhesive foam strip. 2mm thickness is about right.
I'd use screws on the rear panel. Plenty of them. I'm assuming you're gluing the panel in, but do use foam gasket otherwise.
Fixing the speaker with T-nuts and bolts (likely M4 or 5) is the preferred method, but woodscrews should work on this size of speaker. If the Fane is already gasketed (which is unusual), you won't need to add any more.
If you really need silicone sealant, you haven't built the cab properly.
Unless you have a good (i.e. expensive) crimp tool and the correct crimps (there are different specifications), solder the terminals. It's fine.
You don't normally need removable panels to access the inside of the cab, as you should be able to gain access through the hole in the baffle for the driver. By all means, make the panels removeable for testing, but once you're happy with the cab, I'd glue them in.
Sealant has no place in a speaker cabinet!