it is written that Jonas's community cannot see colour, but people can see the colour of Jonas. How?

Question

People in the Jonas's community sees Jonas's eye color

 

Answer 1

Theresa's answer is great. There is more to it though. Jonas has the memory of color, given to him by the Giver. Nobody else in the community has the same memories. in fact, they know nothing of the past, such as color, music, or real love. 

Answer 2

They see the world in grayscale. Like an old black and white picture. Most of the people in his community have dark eyes, but Jonas, Gabe, and the Giver seem to have light (perhaps blue) eyes.