With indexed color images the pixel of an image portions defines which entry is to be used from the palette. If you have an image portion e.g. in the brush container which just has a different color palette then this effect appears.
There are different approaches to overcome this:
if you know that all the colors of your brush are in the color palette of your image or the colors from there are “good enough” then just use menu Colors/Remap Colors/Brush. This will change all pixels of the brush to the best matching color entry of the image palette.
If the brush contains colors that you would like to add to your image then use menu Colors/Import Colors from Brush.
Always be sure to update brush container entries when you change the palette.
I suggest 1. and 3. in this case.
In this video you can see a similar case:
There you use bitmap fonts that may also come with colors not being in the image’s color palette initially.