For a comparison imagine that I have a length of wood that is exaclty 1.006 metres long. If I say that the length to the nearest metre is 1m that is not an error. If I say that the length to the nearest centimetre is 101cm then that is a finer measurement - and still not an error. If I say that the length to the nearest millimetre is 1008mm then that IS an error, despite bieng closer to the true length than either of the previos measurements.
A lack of colour vision is like a less-fine measurement - like measuring to the nearest centimetre instead of the nearest millimetre. It is a loss of discrimination rather than an error.
And it is false to say that errors are impossible. Optical illusions are a genuine example of an error in our sensory systems. However we can tell this because the error is not so completely systematic that it cannot be detected.