I chose yellow and red, using a bright and a more muted version of each.
|Stage 5 Coloured stitches exercise|
Separating the lines of the outer colour (to allow some black through) resulted in the central circle becoming less intense. Surrounding blocks of colour in lines of contrasting colour led to a merging together of the colours. Red squares became more orange, and yellow squares more muted. This seemed to happen more when the central blocks were smaller.
This was repeated in the blocks with alternating lines. Thicker lines intensified the colours of each, thinner ones tending to make them look more like each other.
Altering the relative proportions of the colours - one block was bright yellow with very small flecks of red - this block still looks bright yellow until you look closely. The same was true of another block of the more mustard yellow, which had very thin lines of the darker red between the rows. Only a little more of the contrasting colour is needed for it to appear distinctly - see the diagonal red threads with dashes of yellow.
Different proportions of black had a different effect depending on whether it was bright or muted. The bright yellow appeared more intense with more black in the surrounding area. The crimson red stood out less, but the scarlet was stronger. (See the block with a spiral of red chain stitch with single lines of yellow in the middle section.) The contrast between red and yellow often appeared stronger when there were some black areas left in between them.