Zoysia grass remains tan in color until nighttime temperatures remain above 50 degrees Fahrenheit consistently. Depending on latitude, that means during the period anytime from mid- to late spring. The warmer the spring, the earlier the grass comes out of dormancy and greens up.
Irrigating and fertilizing in winter and spring does not prevent zoysia grass from turning tan in color. However, a moist soil in spring does encourage new green leaf growth in spring's warmth when compared to a dry soil. A cooler than usual springtime weather regime always delays the lawn's green-up.
Not all types of zoysia grass green up in similar fashion. Variety Emerald is the standard used for comparison for newly developed varieties of zoysia grass. In general, Emerald is slow to green up, usually not until late spring. El Toro, in contrast, tolerates cooler temperatures better and thus greens up slightly earlier in spring and remains green longer in autumn.