First, we just looked at the darkest color of 4 different colors, and did grouping the similar structures of the color. However, we then realized those can be rotated. Those grouping are so messy and bothering finding out the identical ones from there.
Actually, we a kind of noticed what it is going on here. The combination of outside color and inside color should be very important. So, we tried grouping the same combinations such as white id outside, and gray is inside (The real puzzle shown in monitor was vivid color, but this is printed.) Then, after grouping, we looked at the other combinations next to what we just classified. Thus, we got it!! This is the best way to find out.
Next puzzle is... Which of these windows can be drawn by one line.