Wen pairing two or more colors together, some "clash" or cause a effect visually where the two colors meet. To me it appears to be a black line between the colors or shading when in fact there is not. I grey scaled the picture to confirm. In dogs, they would just see different shades of grey and this effect would not affect them. So, what is this that which make this effect in the brain?
Magnify your image.
I just put it in GIMP and it is clear that there are several pixels in the red and the blue near the border which are a darker color than those adjacent - the drawing program must be responsible.
Lateral geniculate nucleus
That said, differences of red and green hue are handled more in the parvocellular layers of the LGN, while grayscale differences are handled more in the magnocellular layers, with blue cones contributing to the koniocellular layers the most. In your images, all three are involved. There are some individual differences reported between these regions and their timing of activation in regard to dyslexia - it's a hot topic, and these variations seem to be the explanation for that perplexing condition. I'm not going to point to any specific paper because I'm not sure how to interpret the "clashing", but there's some interesting reading there!