[How would I recolor a blob?]

So I made a blob.

It has different shades of the same colour (green).

I want to change all the shading to blue (with blue shadings).

How would I do that?

I used to know from a tutorial, but I think it was for a different program.

I just need to change the green shading and colour in blue shading and colour.

Is there I tool I could use?

Or do I have to recolour it all over again?

[How do I deselect selection?]

OK. I used the lasso selection tool and I did a linear gradient in the selection. I've finished the gradient, but the selection won't come off. Like the dotted line is still moving around the selection. So I was wondering if anyone knows how to deselect the selection (I find it annoying when I'm trying to add something to the image).