Although there are many optical illusions out there, I get tricked and feel kind of disheartened every time.

Here is one we found from Kyoto Institute of Technology:

Do you see what I am seeing? I see sky blue and light green colors forming a spiral that goes towards the center.

Apparently, the two colors are actually identical. Here's a close-up of the image:

You might still see two shades of light green, if you look carefully enough, it's the same color.

This illusion is based on 'Munker Illusion' and has been around for some time. It never ceases to amaze me how human brains process color this way...