The following image is made of only 3 colors: green, orange, and purple. The blue color you perceive is an optical illusion.
Here is the same image, but the purple layer has been turned off, so all you see is the green and orange. Go ahead. Zoom in on the top image. The "green" and "blue" colors in the top image REALLY are both green.
If you need more proof, download the layered Photoshop file (PSD) here. (Be sure to Save File As...)
