I’m colour blind* (like 8% of the male population in Europe, so every 12th man) and can’t even figure out the proper colour of the arrows. This is where simple black U and V small letters next to the surface edge (or next to the last of the arrows) would help immensely.
Along with “Rebuild surface UV”, there are various other commands that require the user to choose between U and V direction, such like “Rebuild surface” (where the user needs to see both U and V directions simultaneously), “Split” (splitting a surface with the “Isocurve” option from the command prompt) etc. Clear U and V letter indicators will make using there commands much more easier.
By the way, as we discussed in several other topics recently, in future Rhino releases the TAB key should be used to swap the U and V direction in some commands. These commands include: “Split” (again, by the “Isocurve” option), “Rebuild surface UV”, “Insert/remove a control point”, “Insert knot” and others.
PS: Since my colour blidness combines two types, I have a very limited perception of coloured objects that are too small or thin, such like the red and green arrows that appear during the “Rebuild surface UV” command. The problem increases when the red or green are lying on a grey background, because in this situation my eyes see dark halos around the thin red and green arrows. I have a better chance to figure out what’s the colour if it has a much larger area - this is related to the eyes that need more light information to distinguish the different wave length for each colour. For the World axis in Rhino’s viewport I changed the colours to normal red, green and blue (255, 0, 0 / 0, 255, 0 / 0, 0, 255 respectively) and also made them thicker so that I can have a better chance to actually see them are those colours. However, the arrows of the “Rebuild surface UV” command are so thin that sometimes I see the red and green ones like red, green, grey, brown, orange, grey etc. It’s not easy to rely only on those coloured markings in my situation due to the colour blindness. But even if my vision was perfect, I would still make that proposition to ask for adding U and V markings, because this is what will make Rhino even better and more user-friendly.