To work at sea you must have a current certificate of seafarer medical fitness.
The UK Maritime and Coastguard Agency (MCA) approves two types: ENG1 and ML5.
To check you are eligible see Seafarers medical certification guidance from the MCA.
Follow that link and you will find a list of conditions that may prevent the issue of your certificate or restrict its validity.
Most crew jobs require an ENG1 medical.
Commercial Endorsement for Yachtmasters
To work as a professional Yachtmaster you need either ENG1 or ML5 to support commercial endorsement of your qualification. Flying Fish recommends ENG1, which validates Yachtmaster Offshore to 150 miles from a safe haven and Yachtmaster Ocean on an unrestricted basis. ML5 limits you to 60 miles.
The ENG1 medical is available from a restricted list of MCA approved doctors for a maximum fee of £80.
ML5 (MSF 4112)
The MCA provides a form MSF 4112 for the medical known as ML5, which can be completed by any doctor in the UK. Your doctor sets the fee, probably about £120. With this medical a Yachtmaster qualification is valid up to sixty miles from a safe haven.
Download the MCA form MSF 4112 for ML5 and take this to the doctor conducting your medical examination.
If you are colour blind, affecting your ability to distinguish lights, you should consider having an eye test before joining a Yachtmaster course or training as professional crew.
Any doctor can conduct the Ishihara test and if you pass you can be confident that colour blindness will not be a bar to the issue of an ENG1.
If you fail Ishihara you can consider having a "Lantern test" at one of the MCA offices in the UK. The Lantern test is calibrated to the specification required for ENG1. You may be able to pass this test, depending on the nature of your colour blindness.