Straight from the API instruction booklet that comes with the test kit...
"Read the test results by matching the test solution against the *** Color Chart. The tube should be viewed against the white area beside the color chart. Color comparisons are best made in a well-lit area. The closest match indicates the ...."
The *** is for pH, hi pH, ammonia, nitrite and nitrates. It is the same for all of these tests.
You hold the tube against the card. Much like you would hold a sample of tank water against a white background to see a tint to water to determine color of algae.