Get access to custom drawable in watch face view class

There are a few steps necessary to get what you want. I'm assuming that your app looks something like this to start...

class MyWatchFace extends WatchUi.WatchFace
{
    function initialize() {
        WatchFace.initialize();
    }
    
    function onShow() {
    }
    
    function onLayout(dc) {
        setLayout(Rez.Layouts.WatchFace(dc));
    }
    
    function onUpdate(dc) {
        // handle any changes to drawables in the layout
        
        View.onUpdate();
        
        // draw anything on top of the layout
    }
    
    function onHide() {
    }
    
}

class MyApp extends Application.AppBase {

    function initialize() {
        AppBase.initialize();
    }
    
    function onStart(params) {
    }
    
    function getInitialView() {
        return [ new MyWatchFace() ];
    }
    
    function onStop(params) {
    }
}

The first steps are getting access to the background drawable, getting the property value, and setting the color. The View object has a findDrawableById() method that can be used to get a drawable. So this part is pretty easy:

// snip...
    
    function onLayout(dc) {
        setLayout(Rez.Layouts.WatchFace(dc));

        // read the property
        var color = Properties.getValue("BackColor");
        if (color == null) {
            color = Graphics.COLOR_BLACK;
        }

        // find the drawable
        var drawable = findDrawableById("BackDrawable");

        // set the color        
        drawable.setColor(color);
    }

Here I'm updating the color from onLayout(). This will update the color after the properties have changed and the watch face is been re-started (enter the widget loop and return). You could also put that new code into onUpdate() or onPartialUpdate() to update the color more frequently. That might seem good at first, but the screen updates at least once a minute, and typically the setting isn't changing every update, so that would be a lot of wasted cycles over a day. We can do better.

The AppBase object has an onSettingsChanged() callback that is invoked by the system when settings have been changed externally (Garmin Connect Mobile, the ConnectIQ app, or Garmin Express). You can update your AppBase object to keep a reference to the WatchFace, and then use this to notify the view to refresh itself when the settings have changed.

class MyWatchFace extends WatchUi.WatchFace
{
    hidden var _settingsChanged = true;

    function onLayout(dc) {
        setLayout(Rez.Layouts.WatchFace(dc));
        
        // read the settings from storage on initial display
        applySettings();
    }
    
    function onUpdate(dc) {
        if (_settingsChanged) {

            // read the settings from storage because they have changed
            applySettings();
        }

        View.onUpdate();
    }
    
    function applySettings() {
        // read the property
        var color = Properties.getValue("BackColor");
        if (color == null) {
            color = Graphics.COLOR_BLACK;
        }

        // find the drawable
        var drawable = findDrawableById("BackDrawable");

        // set the color        
        drawable.setColor(color);
        
        // we've appllied settings, clear the flag so we don't update
        // unless necessary
        _settingsChanged = false;
    }
    
    function onSettingsChanged() {
        _settingsChanged = true;
    }
    
}

class MyApp extends Application.AppBase {

    hidden var _view;

    // snip...

    function getInitialView() {
        _view = new MyWatchFace();
        return [ _view ];
    }
    
    function onSettingsChanged() {
        if (_view != null) {
            _view.onSettingsChanged();
        }
    }

    // snip...
}

Another option here is to just skip using layouts and do it all with dc calls.  It's something to look at if you're getting tight on memory.

You only need to worry about settings in two places - when the app starts and in onSettingsChanged.

Then in onUpdate, all you need is

dc.setColor(MySettings.background,MySettings.background);

dc.clear();

dc.setColor(MySettings.textcolor,Graphix.COLOR_TRANSPARENT);    //set color used for the foreground

And then just reference MySettings in onPartialUpdate if that's being used.