If it's a drawing application, then I recommend doing something like this:
ACLS
XSCREEN 3 'no 3d, hide keyboard
DISPLAY 1
COLOR=RGB(255,0,0) 'red
WHILE 1 'Infinite loop
VSYNC
OLDX=TCHX:OLDY=TCHY
TOUCH OUT TCHT,TCHX,TCHY
IF TCHT==1 THEN
GPSET TCHX,TCHY,COLOR
ELSE
GLINE OLDX,OLDY,TCHX,TCHY,COLOR
ENDIF
WEND