As usual, this topic can be about as complicated as you want to make it.
Lots of different treatments have been applied to the underwater hulls of ships and boats over the centuries. During the seventeenth century the most common probably was indeed a mixture of white lead and tallow, which would have a cream or off-white appearance. That seems to be what's shown in most of the old master marine paintings, and the few models of the period that have planked bottoms.
It probably isn't as simple as that, though. Brian Lavery's The Arming and Fitting of the Sailing Man of War (I may have the title garbled a little), which is an excellent, modern resource on such stuff, asserts that there were two basic types of hull treatment prior to the introduction of copper sheathing: "white stuff" and "dark stuff." "Dark stuff" apparently was a helllish concoction made of tar, sulphur, and probably some other awful stuff that was designed to discourage various forms of marine life. (It certainly would discourage me.) I don't have Lavery's book in front of me at the moment, but I believe he suggests that "dark stuff" was actually more common. I think he also has an illustration of a ship with a fairly broad light stripe stretching a few feet below the waterline, with "dark stuff" from there to the bottom of the keel.
I'm not sure I'd ever want to build - or look at - a model of a seventeenth-century ship with an accurately-rendered bottom. Seaweed, barnacles, and all sorts of other crud must have stuck to it.
Bottom line: the off-white bottom, perhaps with some weathering applied, is a reasonble compromise between accuracy and avoidance of the visually disgusting. That's how I handled the bottom of my last big project, the Revolutionary War frigate Hancock.
Also on the topic of bottom colors: recent research has established that the "copper bottom" is also the subject of some discussion and variation. The first "copper sheathing" (dating from the time of the American Revolution) was indeed copper (or "red metal," as they called it in those days). In about the middle of the nineteenth century, though, the dealers started mixing the copper with zinc and/or tin, producing a considerably tougher, better wearing material known as "yellow metal" (i.e., brass). For a nineteenth-century clipper ship (e.g., a Flying Cloud or Thermopylae), brass probably would be a more accurate bottom color than copper. (The one surviving example, the Cutty Sark, is sheathed with something called "Muntz metal," which is also similar to brass.) My ongoing big project is the clipper Young America, from 1853. I've been stewing for some time over how to handle her bottom. I've got some sheet brass that would work, but it would look a little odd. And copper is so pretty....