Tan lines.

Tan is not a good colour descriptor for wine; tawny for a port, perhaps, but definitely not sunburn for a Cabernet Sauvignon (CS). However, the sunburn issue here, rather, is heat damage on CS grapes not the hue of a finished wine.
The Napa Valley experienced a rather cool spring (I think I've mentioned that before), coupled with the elevated moisture content of the soil, due to California's excessively wet winter, the grapevines have had a fine start to the 2017 growing season.  In such conditions the vines tend to experience a period of rapid growth, putting all their energy into leaf, shoot and grape making.  A sudden hot spell, like that which arrived in the second week of June, caught the vines unawares and they were not well prepared to deal with, or quick enough to react to, the ensuing heat stress.
The heat damage to the photographed cluster of CS grapes is minor and occurred due to the fact that that particular grapevine is always a little weak and thus cannot produce a better canopy with which to shade the immature clusters.  It is likely that this cluster will raisin and I'll have to drop it (and any others displaying similar heat damage). Wine made from fruit that is sunburnt can be bitter, raisin-like and have issues with VA; all of which are characteristics found in bad wines, made from poor fruit, that are persona non grata in my glass.  So, off with their heads.