Compass roses date back to the 1300s and were originally designed to show the wind directions. This is why you'll see different abbreviations at times from those we are accustomed to in the 21st century. The names designate the various winds in Italian, and labeling sometimes varied from mapmaker to mapmaker. For example, the "tramontana" wind, which comes from due north, was sometimes labeled with a "T" instead of an "N."
On many maps today, you only see one letter on the compass -- the "N"pointing toward north. With early compass roses, a spearhead or fleur-de-lis (starting with Portuguese maps when Columbus was preparing to find a new way to India) often served as the marker for north (or "tramontana"), and navigators were expected to know the other 31 points based on that one.
Colors were used to add beauty and contrast to the images on the compass rose on maps during the ages of exploration and empire. Mapmakers took a great deal of pride in their artwork, adding such details as mythological creatures in the deep seas and vivid coloring in the compass rose. Another purpose for the colors was to make the compass easier to read. Navigators had to read maps at night, in the weak glow of lanterns, and having markedly different colors for compass directions was helpful.
Rhumb lines often radiated out from the different points on compass roses to help navigators determine bearings on maps. On a rolling ship, lines could be difficult to gauge and follow precisely. Most maps indicated north (and still show it) as pointing toward the top of the map, although smaller maps of specific landforms or bodies of water sometimes altered the direction for ease of use.