Is it normal for the colour of the cooked lentils to change overnight even when they were kept in fridge? Last night we cooked the lentils in a pressure cooker 20:00. They were on the kitchen shelf for about 3 hours before I put them in fridge.

I took them out in the morning at 9:30, and took them to office on scooter (It takes 30 minutes to reach office). Day time temperature here is 35 C.

Office has AC switched on. I opened the box of cooked lentils at 14:00 in the office and found colour of the lentils quite a pale green. That is not normal for properly freshly cooked lentils. 

I stirred the lentil box with a spoon and found that the lentils below the surface were of normal green colour.

Is it normal for the colour of the cooked lentils to change overnight even when they were kept in fridge?
the color change you've observed is from oxidation. If you want to prevent this, lay some plastic wrap over the surface of your lentils to prevent air from getting to them, or add some acid, like lime juice, to the recipe.