Pool water tends to give off a slightly blue tint when properly maintained. If you have noticed your pool taking on a more green tone, you probably don't have the proper level of chemicals and perhaps are not cleaning your pool regularly. The green color is algae. To get the blue color back, you need to change the balance in the water.
Clean the pool to remove surface debris such as leaves. Vacuum the bottom and sides with a pool vacuum at least once a day.
Clean the pool filter by following manufacturer's instructions on opening the filter housing, removing and rinsing off all the filter fingers, then adding new diatomaceous earth (DE). Ensure pressure readings are in the range indicated in the instructions.
Test the water once a week to see what the chemical levels are. Water test kits are available at pool stores and the pool sections of super centers. Fill the tubes with pool water and follow directions on the kit for proper readings and levels.
Add acid or alkaline chemicals as needed based on the pH test. If the pool is severely green, you will want to shock it with 3 oz. of chlorine for every 1,000 gallons of water. Run the filter system for most of the day to circulate the contents.
Set up an external pump system to filter the water in extreme cases of algae.