For reasons best known to Mathworks appdesigner gives public access to all UI components created so you simply access the relevant UI component and do whatever you want with it. It's horrible from a code design perspective, but appdesigner lets you do it easily.
e.g. in app1, so long as you have access to app2, having kept its handle when you launched it:
app2.myStaticText.Color = 'r';
or whatever you wish.
