R0 Number for Formula 153, Formula 154, and Formula 155
Most Formulas in the MIL-DTL-24441 specification match a specific color code in Federal Standard 595.
However, Formula 153, 154, and 155 are defined instead by the amount of light reflected from the coated surface, with a wet film covering, with an illumination angle of 45 degrees from the vertical.
The lower the R0 number, the darker the coating, as more light is absorbed rather than reflected.
Details from the MIL-DTL-24441 Specification
Here's the relevant portion, from Verification, of the MIL-DTL-24441 specification.
4.5.7 Color. Determine color in accordance with one of the following, depending on whether wet film or dry film is specified.
4.5.7.1 Camouflage coatings. Wetted film for camouflage coatings (camouflage coatings are identified as formulas 153, 154, and 155). Prepare the test panel by spraying on smooth steel a 100-µm (4-mil) (nominal) wet film thickness of material prepared as specified in 4.5.1. Dry the panel for 16 hours at ambient laboratory conditions, then 15 minutes at 52 ±3 °C (125±5 °F). The color shall be measured using ASTM D2244. Color shall be in accordance with the requirements of the appropriate specification sheet.
4.5.7.1.1 Reflectometer. The reflectometer shall have a geometry with an illumination angle of 45 degrees from the perpendicular, and a viewing angle of 0 degrees from the perpendicular. The reflectometer shall measure green reflectance in the range of 0.80±0.01 to 4.00±0.01 percent. Any instrument with the required precision and accuracy is suitable for use (see 4.5.7.1.4).
4.5.7.1.2 Wetting solution. Distilled water containing approximately 0.1 percent of a clear wetting agent (detergent) shall be used for wetting the specimens. Solution shall wet the paint film in a sheet without foam.
4.5.7.1.3 Procedure. Cover the specimen to be tested with a thin film of the wetting solution mixture (see 4.5.7.1.2) and immediately obtain a green reflectance reading. Check specimen after obtaining reading for intact water film. If water film is not intact, repeat procedure until a satisfactory reading is obtained on a fully wetted test area.
4.5.7.1.4 Alternative color measuring equipment. Manufacturers can qualify alternative color measuring equipment to NAVSEA or the General Services Administration (GSA) by demonstrating that the particular instrumentation to be used measures the green color parameter with the accuracy specified in 4.5.7.1.1. The manufacturer shall demonstrate that the alternate instrumentation can provide green color parameters that result in the colors defined in the appropriate specification sheet.