Reducing numbers of colour in GIMP is not helping me

[simon1]

Hi, I am trying to reduce the number of colours from over 17k to just 7 colours to Black, White, Blue, Yellow, Red, Light Grey and Dark Grey. If refer to the attachment, you may get a clearer picture. I am being bothered by those colours exist in between two colours which does not match with any of the colours. I have tried everything available in GIMP but could not solve the problem. Is thee any other way? Thanks.

[Ofnuts]

No escape These pixels of mixed colors are what makes the edges look smooth (see here for an explanation). The more colors you remove to jaggier the edges will look.

Which doesn't mean you need to keep all of them, you can possibly find a balance. But instead of letting Gimp decide which ones to keep (possibly in the basis of count), you can perhaps create a palette with your 7 colors and some possible blends(*), and specify it when using Image>Mode>Indexed.

(* )Technically you have 7x6=42 possible color frontiers on edges, to spread over 256-7=249 colors (256 colors max minus the 7 colors you want). I think you can generate them easily (including you 7 base colors) using a trick like this:

   

and then importing the palette from such an image.