How could I remove the grey parts in the background? I hope everyone has had a nice day]1
Right click on the Eraser tool and select the Magic Eraser.
Click once or more times on the area you want to remove.
If the area becomes transparent (white and gray checkered) fill it with the desired color using the Paint Bucket tool.
Use Pen Tool
Ctrl + P or Magic Wand Tool
Ctrl + W