Hi,
To activate grids/groups when an OSC message is received, you can use a formula like this for the activation slider:
trigger(osc.onreceived(0), 10 /* time in seconds */, 0, 100, 0)
To overlay effects, you set the precedence of the effects that you want to take over to a higher value. For example, you set the precedence to the grid for the all red effect to 10. So when it's activated, it will override all effects with lower precedence and when you'll deactivate it, then the lower precedence effects will be still be playing. You set the precedence of grids in the grid manager (view/grids menu and then select a grid).