There are five primary color-tests reagents used for general screening purposes. The Marquis reagent turns into a variety of colors when in the presence of different substances. Dille-Koppanyi reagent uses two chemical solutions which turns a violet-blue color in the presence of barbiturates. Duquenois-Levine reagent is a series of chemical solutions that turn to the color of purple when the vegetation of marijuana is added. Van Urk reagent turns blue-purple when in the presence of LSD. Scott Test's chemical solution shows up as a faint blue for cocaine base. 
Next, you take a Civil Service Parole Officer Sample Practice Exam. Here, you work with all of the various question types and topics by completing the exam. Then you can cross reference your answer to the provided correct solutions provided in the Answer Key. You are given explanations for each question, thus you can determine where your strong points are and also where your weak areas are and then go back through the guide and review the problems that gave you difficulty. This method of preparation helps you zero in on your weaknesses, discover how to overcome them and thus boost your score because you will have mastered the material.