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?