Not sure what you are doing ie. As soon as I switch to the thought layer the selection area changes Color Select not the best tool for this.
You have your background image - the bubble
You add a layer on top - something with a transparent background.
Make the selection using the background layer, fuzzy select usually works.
example: https://i.imgur.com/SNHmZsh.jpg
Switch to the layer you want to trim.
Invert the selection, Select -> Invert then cut the selection Edit -> Cut
example: https://i.imgur.com/94jy9EU.jpg
You have your background image - the bubble
You add a layer on top - something with a transparent background.
Make the selection using the background layer, fuzzy select usually works.
example: https://i.imgur.com/SNHmZsh.jpg
Switch to the layer you want to trim.
Invert the selection, Select -> Invert then cut the selection Edit -> Cut
example: https://i.imgur.com/94jy9EU.jpg