You can visit the MUTCD website www.mutcd.fhwa.dot.gov

MUTCD Colors:

The following color chart establishes general meanings for colors that have been identified as being appropriate for use in conveying traffic control information. Central values and tolerance limits for each color are available from the Federal Highway Administration.

The color specifications used in traffic control were developed in the late 1960's and for the most part remained in use until recently. The technological advances in the manufacturing of signing and markings materials and the measurement of color required the FHWA to revise and expand the color specifications. This revision includes daytime and nighttime specifications for both assigned and unassigned colors found in the Manual on Uniform Traffic Control Devices. 

MUTCD Language:

"shall"

Mandatory condition. Where certain requirements in the design or application of the device are described with the "shall" stipulation, it is mandatory when an installation is made that these requirements are met.

"should"

Advisory condition. Where the word "should" is used, it is considered to be advisable usage, recommended, but not manatory.

"may"

Permissive condition. No requirement for design or application is intended.