Mouse = use for drawing 
Mouse Left Button = create sand particles 
Mouse Right Button = remove sand particles
Mouse Wheel = change the size of the brush
Mouse Center Click = pick up the current color at the mouse pointer  
N = change sand color to a random color
S = save current image as a png file
B = toggle paint mode 
C = clear the screen
R = toggle random color mode
I = toggle information 
U = undo the last action
T = toggle static mode
P = toggle plain color mode
L = open the color picker
G = toggle brush randomness