Some of the vendor websites have a way of overlaying various wheels on pictures of your car's year/make/model. If you've done exterior mods, it won't be exact, but it'll still give you a pretty good idea.
Or you could do the same thing yourself with pictures of your car and pictures of the wheels scaled to suit.
FWIW, I'd probably pick silver or chrome in that order, or perhaps silver, matte or gunmetal (what your car's spokes appear to be), chrome. Not black with that white "M U S T A N G" stripe and the tri-bar.
45 and 40 are aspect ratios, it defines how tall the sidewall is compared to the tire width, so if you have 285/40, the sidewall height is 40% of 285mm, so 114mm or about 4.49", on a 255/45, the sidewall height is 45% of 255mm, so 114.75mm or about 4.52"... so you're telling me you can eyeball a difference of 3 hundredths of an inch?
Upon further review it looks like AMs staggered pachages use 275/40 on the 10's and so the difference of about 1/5" in sidewall height may be noticeable.
275/40 and 255/45 is a noticeable difference, yes.