KONAMI

You can wear camouflage to blend in with your environment and deceive your enemies. Select "Camouflage" in the Survival Viewer and pick camouflage that matches your current mission area to increase your ability to blend in.
To camouflage yourself, use "Uniform" to change outfits, and "Face" to apply face paint.

The Camo Index

To see how well Snake has blended in with his surroundings, you can check the Camo Index in the upper-right corner of the game screen. The top indicates his current face paint, while the bottom shows his current clothing. The higher the Camo Index, the more effective Snake's camouflage will be, making it harder for him to be spotted by enemies.

No matter how good Snake's camouflage is though, his Camo Index will drop considerably while he is moving. Therefore, his body positioning also has a large effect on the Camo Index.

When the enemy is nearby, try to get in a position with a high Camo Index and refrain from moving.
You can keep your Camo Index low by stalking an enemy.