A couple of minor points from a certifiable Olde Phogie.
First - I'd really be interested to know where the idea that the clove hitch is a "difficult knot" got started. As David has found out, it's just as easy to tie as the square (reef) knot - if not easier. That assertion has nothing to do with experience or stodgy attitudes; it's a simple statement of fact. I'm prepared to assert that anybody who can tie a square knot can tie a clove hitch. There just isn't any rational reason to use reef knots instead of clove hitches on ratlines. If anybody out there genuinely thinks the clove hitch is a difficult knot, I'd be curious to know why.
Second - the subject of how ratlines really look is an interesting one. First a little vocabulary is in order. A ratline and a footrope are two very different things. A footrope is a line that hangs down from a yard, to support the men who are working on the sail. A ratline is a piece of line that's tied to a set of shrouds (with clove hitches) to form a ladder for men climbing aloft. To my eye, David's ratlines look about right.
The shrouds are among the heaviest lines in the ship. They keep the masts from falling over, and transmit the force of the wind in the sails to the hull, thereby pulling the ship through the water. The ratlines only have to support a man's weight, so they can be much smaller - typically about half an inch in diameter. On a small-scale model you're unlikely to make the ratlines too fine.
The old master marine painters in the days before photography generally showed some "sag" in their ratlines. To some extent the slack was desireable; it gave a man a slightly better footing. Photos of latter-day sailing ships vary in the amount of slack they show; I've seen some that look downright droopy, and others that look as though somebody set them up as taut as he could. (Beware museum ships, and photos of them. Their rigging - which is probably made from synthetic line - is often dubious.)
In the last days of sail ratlines were often made of iron rod, which can be counted upon to be really straight.
There's room for some argument about the color of ratlines - and, for that matter, the rest of a ship's rigging. Generally speaking, in the days before wire rigging the standing rigging, including the shrouds, was coated with a concoction containing tar and lampblack, which, if it wasn't pure black, must have been close to it. Some modelers assert that ratlines ought to be natural rope-colored, on the theory that tar would make them stiff and slippery. Maybe those modelers are right, but I have yet to see a photo or painting that shows ratlines paler than shrouds. The website of H.M.S. Victory includes a "modeler's page" and a detailed color scheme. It says the Victory's shrouds are blackened, and the ratlines are "lightly tarred." That makes sense to me.