You say According to our present understanding the two are related by the physical speed of light this is true but it is truer than you think according to any reasonable understanding wavelength is related to speed Suppose for example it was found that light travelled slightly faster in an east west direction than a north south direction unrealistically then light would travel further in a given time in an E W direction than it would in a N S direction in the same time but the wavelength would also be shorter in the E W direction The wavelength always depends on the speed whatever rules apply to the speed