5 Colo. Code Regs. § 1001-15-A-VII
Subject to the provisions of Section 42-4-413, C.R.S., as amended, and Section 42-4-414, C.R.S., as amended vehicles inspected following the procedures established in the Part A of this regulation shall meet the following opacity standards. The smoke opacity standard for all diesel vehicles subject to loaded mode opacity test, under Part A, IV.C.1. through IV.C.4. of this regulation shall be thirty-five percent (35%) and twenty percent (20%) for naturally aspirated and turbocharged diesel vehicles respectively for five (5) seconds. The smoke opacity standard for all diesel vehicles of model year 1991 and newer subject to snap acceleration J1667 opacity tests, under Part A, IV.C.5. of this regulation shall be 40% opacity. The smoke opacity standard for all diesel vehicles of model year 1990 and older subject to snap acceleration J1667 opacity tests, under Part A, IV.C.5. of this regulation shall be 55% opacity.
5 CCR 1001-15-A-VII