Here, three sets of different coloured glasses are provided with a sliding mechanism. By sliding the glasses, merge one colour with another and view through them.

What did you observe? 
You see a different colour for each mixture.

The science behind it :
Red, Yellow and Blue are known as primary colours. The resultant colours for each mixture are known as secondary colours. When blue and red are mixed, we get purple.When red and yellow are mixed, we get orange. When blue and yellow are mixed, we get green. Hence, purple, orange and green are known as the secondary colours.