A two-color diagram of (FUV − B) vs. (B − V) for field stars from Sierchio et al. (2014), Ballering et al. (2013), Isaacson & Fischer (2010), and Lorenzo-Oliveira et al. (2018). Duplicate stars are not shown. Significant differences in (FUV − B) at any given (B − V) indicate differences in chromospheric FUV emission levels. The red line denotes an upper envelope constraint used to demonstrate a minimum level of chromospheric activity.