Issues with drawing inside vector

When I try to draw inside a vector I made, the colour of the vector changes and I'm not sure what I'm doing wrong.

Here's what I've done

I have created a vector with a gradient.

I press the vector and choose draw inside.

I click outside the vector.

I choose the brush tool and change to the colour I want (sometimes I've seen the colour of the vector change already at this step).

I draw inside the vector.

The colour of the vector changes. 

I have also recorded a video so that you can see what happens.

https://www.loom.com/share/412df7d31a3f43e6b55e9112b93cccb9

Thankful for help!