Try using a task function to control the program flow. In the following code the screen will turn white for 0.5 seconds after the 's' key is pressed:
Code:
import viz
import viztask
viz.go()
def whiteScreen():
print('White')
viz.clearcolor(256, 256, 256)
vizact.onkeydown('w', whiteScreen)
def blackScreen():
print('Black')
viz.clearcolor(0, 0, 0)
vizact.onkeydown('b', blackScreen)
def signalTask():
while True:
yield viztask.waitKeyDown('s')
whiteScreen()
yield viztask.waitTime(0.5)
blackScreen()
viztask.schedule(signalTask())