Hello, this is the tutorial using opacity mask, like the title said. Using this mask, you could have a more complicated mask , and I think this is an awesome trick to learn. Let’s get it done then….
Step01:
Make box using rectangle tool(M), then go to window>swatch library>gradients>sky. Fill it without any stroke.  And we just made our base color.

Step02:
Next, go to window>symbol>grime vector pack. Select symbol sprayer tool (shift+S) and then spray it once.

Step03:
Select the sprayed item, go to object>expand. Click ok.

And then recolor it into white. Drag it into …