Fur patterns are not hard to implement--the current status effects for both Vanae Markings and Shark Markings utilize the skin accent color as the secondary color and the marking patterns are either ambiguous (for vanae) or defined by a value (for shark: stripes, spots, blotch). So if custom markings are ever to be implemented they need to define the marking's color (which may have to overwrite the accent color). Any complex color combinations or patterns should probably be avoided however, as that may make the description wordy or color parsing inconsistent (simple is best--the player can fill the rest in with imagination).
Keep in mind that any markings that cover the body (with the exception of skin freckles, since that is a skin flag instead of a cosmetic status effect) will need to remove the previous markings (like gaining Shark Markings will erase Vanae Markings if the player character had them initially). Any new patterns will require an appearance blurb as well. Currently, patterns will not show up in color parsers, so the markings are only ever noted in the appearance screen.
Alternatively, if the markings you want to create are not exclusively covering the body but you want to restrict them to just the fur/scales/feathers/etc., then it may be best to have it replace the fur/scale/etc. color entirely (like fur/feather color is "black and white", or scale/chitin/bark color is "blue spotted yellow", or skin color is "red stripped gold", etc.). The only issue with this is that every time the color is mentioned, it will be the entire string of text. Any transformation effect that changes the color of the area will overwrite that pattern. And yes, fur and feathers share the same color (fur color); scales, chitin, and bark share the same color (scale color); and skin, latex and other skin types use the same color (skintone).
So tl;dr is: Implementation is not difficult, it just depends on how you want it to be specifically. Either they are replaceable body markings or a specific color string.
