I'll explain it in simple terms, those wanting a deeper explanation can no doubt Google it.

In the colour spectrum, all colours have a complementary colour; an 'opposite' in other words. Add these 2 colours together & they cancel each other out.
If all colours were mixed together they make a white light. This is proven by looking at how light refracts through a clear prism; the white light that goes into the prism is 'bent' & all colours 'bend' at a different degree, thus making the rainbow coloured spectrum we see out the other side of the prism.
You see, the colours we see are actually the reflected light off the surface, all other colours of the spectrum are absorbed into the object.
Pink & green are complementary colours.

So your eyes are fixated on the ink; the brain says 'whoaa too much pink there!" and compensates drastically by 'seeing' the opposite colour. Further proof of that is when you look at a bright white light & then look away; you see a black dot. White & black are obvious 'opposites'.
Eventually, your eyes/brain will say 'hang on, there is pink there still and then I see green" till eventually the 2 colours detected cancel each other out.

That was fun and it worked!