Color–color diagram for a sample of WD stars from the SDSS DR7 catalog (Kleinman et al. 2013). The SF18 sample is shown with error bars. Lines denote theoretical color-color curves for blackbody (green short-dashed), pure-He (red solid), pure-H (blue long-dashed), and mixed H/He (dotted) atmosphere models as indicated in the figure legend, in the range 14 000–7000 K. Small circles on top of the curves show blackbody, He atmosphere, and H atmosphere models with marks indicating 1000 K intervals. The circles on the bridging between H and He models mark the log(N_H/N_He) = −2, −4, −6 values.