Nylgut is brighter?
Probably should stay out of this, since neither of these are Southcoast Strings, but I have always found it interesting how many people describe Nylguts as bright or harsh. Just goes to show, I think, the kind of tones your ear picks up.
I always listen to the strings individually, then to how they transition, and only at last as to how they sound together. What you find that way is that Nylgut comes as close as any string ever formulated to the sound of gut. Not surprising, as this was their goal.
The strongest characteristic of both gut and nylgut is how quickly the tone changes
from thin diameters to thick. Thin guages can be as bright as the brightest flourocarbons, but as the diameters increase, the tone softens very quickly. You can use this like Cliff Edwards, for example, putting on a heavy gauge set, then slapping the 3rd string like a bass and playing the others for melody. It will give the effect of two different instruments altogether.
If you have one of the lighter nygut sets, your outsides will be very bright - get one of their heavier sets, however and things calm down in a hurry.
Flourocarbons, go to the other extreme. All single material sets will have softer tones on the thicker diamenters, but with flouros, the difference is much less. The bass strings, in other words are brighter in relation to the trebles. Heavy and light sets maintain a close tonal imprint as well.
I am guessing all the remarks about bright Nylguts are mainly because higher registers are more prominent to the ear and their lighter sets must be the most popular.