Problem with colour pick and changing its colour


I’m new to the Aseprite community. Currently, I am facing a problem with my colour picker. on my layer, I have 2 colours, the text outline and background. When I try to colour pick the background and change it’s colour, or I colour pick the BLACK outline of my text, it will somehow “Merge” these 2 colours together.
I am trying to change colour of the background and of the Text outline.
Much thanks!!