Color lines are thresholds of solar zenith angle for "dat-time".
Vegetation condition is monitored by visible bands, thus observation with sun light is critical. The picture above helps us understand when and where we may have valid observations of visible bands. This is for information only, and there is nothing to do with satellites.
Below is the sun-rise and sunset information for Washington DC. Data was from U. S. Naval Observatory web site (http://aa.usno.navy.mil/data/docs/RS_OneYear.php)