The contour lines are the first in a series of what I'll call "transparent overlay" layers; these are vector data drawn on a transparent background that show up as checkboxes in the overlay selection box. The contour layer is intended to sit on top of aerial imagery; in an environment without trails or roads, combining it with shaded relief can provide better terrain visualization than a traditional topo. The other benefit is that by printing an aerial+contour map and a traditional topo, it becomes easy to match locations between the two.
One difficulty in creating the contour layer was choosing a line color. Traditional USGS brown works great on topo maps because it only has to mix with a couple colors, all light. Aerial images have a much wider range, and a noisier background. The USGS has decided to stick with brown for its "US Topo" aerial-based maps, but I find the lines difficult to follow, especially on prints.
The other issue was line weight. Too thin, and they're hard to follow; too thick, and you start obscuring details in the underlying map. Red seemed like a good bright, unnatural color, but it didn't mix well with CalTopo's SAR origins: autogenerated assignment and debriefing maps use red for the line color. I also found that my brain was accustomed to brown, and that bright colors were harder for the back of my mind to turn into a mental terrain map.
Parts of Europe use orange for their contour lines, and in the end I settled for an orange-brown color. Is it perfect? Probably not. Does it get the job done? I think so. Hopefully you agree.
Top of Avalanche Gulch, Mt Shasta