so, objects - like dresses - exist in an objective reality. light reflects off of these objects in specific wavelengths.

in order to determine what colour an object is, you measure the wavelength of the light reflecting off of it. this is not subjective - this is quantitative data. numbers. facts.

you can then compare what you experience to what the data says in order to determine if your eyes work right or not (preferably double-blind, to control for biases). if what you see is consistent with the data, congratulations - your eyes work right. if not, the mechanics of your eyes are malfunctioning and you are consequently interpreting the world through a flawed filter. you'd want to seek medical attention.