coloured lines convert to shade

[Pysifr]

I am creating a tartan - but the online Tartan Designer shows the colours by different horizontal and vertical interwoven coloured "threads".
While the "shade" created by these lines is realistic, I want the "interwoven" colours to show as a "simple" "flat" shade.   

Is it possible to "blend" the "threaded" colours into a single colour shade? 
   

[Ofnuts]

Filters > Blur > Median blur with a radius of 1:

   

If you have the pixel-synchronized version (threads an exact number of pixels) you can also try Filters > Blur > Pixellize (with a 2x2 thread block).