Merchant Mariners license and colorblindness

In the Coast Guard’s world there are no intermediate levels of colorblindness the test conducted by your physician is a pass/fail examination. If you are unable to pass your color vision test, don’t worry, the U.S. Coast Guard will simply impose a daytime restriction on your license. This just means that you cannot operate a vessel professionally from sunset to sunrise.
From Code of Federal Regs
46 CFR 10.205
(d) Physical examination. (1) All applicants for an original license must pass an examination given by a licensed physician or a licensed physician assistant and present to the OCMI a completed Coast Guard physical examination form, or the equivalent, executed by the physician.

This form must provide information on the applicant’s acuity of vision, color sense, and general physical condition. This examination must have been completed prior to submission of the application and not more than 12 months prior to issuance of the license. (Physical examinations are not required for staff officers.) (2) For an original license as master, mate, pilot, or operator, the applicant must have vision correctable to at least 20/40 in each eye and uncorrected vision of at least 20/200 in each eye. The color sense must be determined to be satisfactory when tested by any of the following methods, without the use of color sensing lenses: (i) Pseudoisochromatic Plates (Dvorine, 2nd Edition; AOC; revised edition or AOC-HRR; Ishihara 16-, 24-, or 38-plate editions).

(ii) EldridgeGreen Color Perception Lantern.

(iii) Farnsworth Lantern.

(iv) Keystone Orthoscope.

(v) Keystone Telebinocular.

(vi) SAMCTT (School of Aviation Medicine Color Threshold Tester).

(vii) Titmus Optical Vision Tester.

(viii) Williams Lantern.

Shop a DR until you can get a certifying letter.

Bottom line – Color blindness, no matter how severe, will not prevent you from qualifying for a USCG issued Captains License