"Simultaneous Contrast" is quite a phrase and quite a concept. I'm in a study group on this topic and I am in awe of how many ways there are to address the issues that it presents.
A fella by the name of Chevreul was the first to present his "law" of simultaneous contrast (among other things). Basically, this means that if two colors are next to each other (and are roughly the same size in area), each will shift as they are are affected by the (near) complement of the other.
So - if you look at red and green, the red looks more red and the green looks more green - since red and green are complementary colors!
It's easy to see if the colors are complmenetary colors to begin with - since they jsut intensify each other.
That's why red&green, orange&blue, yellow&violet are so eye-popping - all because of simultaneous contrast!
If you have colors that are not complementary, it gets a little more complicated.
Keep in mind that one color is affected by the neighbor of the other color's complement - thus the yellow surrounded by red will tend to shift towards a yellow-greenish tinge (green is red's complement and yellow green is nearer to or the neighbor of green when you look towards yellow. If you isolate that little yellow petal, the yellow looks like yellow - if you see it with the others around it - it shifts a tiny bit towards greenish. the red isn't affected since the yellow is so small.
If you don't see that, how about the purple and green succulents - the green should look more yellow-green and the purple should look more red-purple since they are so plentiful and close in proximity.
Again, if you look at one color with the other blocked off and then compare the entire image, the colors do shift a bit.
This is your brain on Simultaneous Contrast!
How about this yellow with orange? The yellow should shift towards blue-green and the orange towards red-purple.
I'm having such fun with this and how it expresses itself in fiber. I'll have some photos of those project soon - they're not yet finsihed enough to make sense in a photo!