Water sensitive paper is a yellow-colored paper card that was developed for quick and easy evaluation of your sprays. It is intended for monitoring spray distribution, swath widths, spray penetration, and droplet densities from your spray applications. It also determines how effective your spraying nozzles are.
The upper side of the yellow-colored paper is specially coated, so it will be stained with dark blue dots when exposed to liquefied spray droplets (the opposite side is water repellent).
Using the Water Sensitive Paper is really easy: put the paper in the desired area before you spray, then after it's been sprayed, allow it to dry and examine the droplet pattern. These papers allow you to monitor your spraying to ensure you are adequately using the right amount of spray — not too much and not too little.