Solid Fill not working as expected

Is is possible that you have a gap somewhere, you have to really zoom in and inspect it I think. But Paul will know better.

Hi Paulo,

I am still a noob, when it comes to GP, but it might be helpful for Paul (or someone else who knows), what Blender Version you are using.

Assuming your cloud is on a separate grease pencil object (the exercise is also to make assets), toggle into Draw mode

Select the Bucket Fill tool, set the material to Solid Fill (if it isn't set already) give it a color attribute, and set it to inverted (the minus sign)

I drew the cloud strokes on a Layer which I named "Lines". I create a second layer and move it below and call it Fills. and make sure this is the active layer

Take a look at these settings for the Bucket Fill:

It is set to Inverted; I have made my boundary strokes visible (this is for demonstration) and I have set the Boundary to "Layer Above".

Now, Click anywhere OUTSIDE the cloud. The first click, you should see all boundary lines marked Red - remember, the Boundary lines will be visible, and you can check before committing.

Now click once more - outside the cloud