If you have too many colours you could reduce the amount by indexing the image. When you index you decide the number of colours to be used.
In your case you have the wall with 8 colours and the door has a lot, so I would:
take (copy) the door alone,
open a new file,
paste the door in the new file and crop the image,
index the image.
I tried two solutions:
The first image is the door you have; the second is indexed with 16 colours; the third with 8 colours.
Then you could work on that and fix it to your liking.
Alternatively you decide the palette beforehand (number of colours, which tones etc.) and draw it.
There's another method: you could use the palette you decided for indexing the image.