If it is a picture you copied into Gravit, it currently still is a bitmap image. You can convert that to a vectorial image via menu option:
- Modify > Path > Vectorize Image
After that you should be able to select parts of the image. For example, to delete the dark blue part or to apply a fill to other parts.
See the image from EllenM’s reply on how to change the fill color. After clicking on the Circle in Fills, you can select a Solid, Gradient or Pattern fill.