In addition to having exciting names, variegated cloths, in my experience, make desirable garments. The distinguishing feature to birdseyes, nailheads, sharkskins and herringbones is that the patterns are a function of weave more than anything else. This differs from something like a pinstripe or windowpane, in which yarns of a different color contrast with the dominant ground color thereby creating pattern. Of course a weave-generated pattern can also employ two or more shades, but the effect still tends to be subtle because the scale is small and the density of the contrast high enough that the cloth blends from even a few feet away.
This really is what is meant by semi-solid, a confounding expression if I’ve ever heard one. The term I prefer, variegated, comes with connotations of irregularity, and I think that is correct. Just as a brick facade might give the impression of a dusty red, random variance in the individual bricks make looking at it interesting. The eye seems to like recognizing tonal arrangements, particularly when, rather than a flat presentation, some dimension is involved. Cloth, like bricks, has dimension, and so reflects light in a dynamic way, enticing the eye to steal second and third glances as the effect changes. Suits in these cloths (particularly at the lighter end of the spectrum) are versatile, tending to look very different from day into evening, seemingly absorbing cues from the surroundings. In fact, a single-breasted blue birdseye might be one of the great staple suits.
Sadly, the versatility isn't equally distributed. Herringbones are perhaps alone in so easily crossing between formal and casual applications. Depending on scale, finish and color, the weave can be found in heavy overcoats, conservative suits, tweed odd jackets—even formal wear. Birdseyes and Nailheads really only seem to work as worsted suiting, but once made up glide easily from conservative settings to more casual ones depending on shirt, tie and accessories. They are excellent travel suits for this reason. Conversely, I can’t imagine sharkskin in anything other than a conservative setting; I’ve seen casual, high-contrast versions, but the effect seems to quarrel with the sober essence of the weave.
These matters are hard to describe though, and even accurate images won’t honestly convey character. This is likely why all those apps intended to help coordinate suit, tie and shirt are always a failure; a screen just can’t replicate the liveliness and dimension of real cloth. Old Apparel Arts issues understood this, often coming with swatch clippings pasted directly to the illustrations. This is a charming, low-tech solution, but in my experience there is no substitute for spending an hour with a comprehensive cloth book. Just try and keep all those colorful names straight.