We don't. For starters there are colourblind people, for seconds there is no specific colour that can be called "red". There is a large and fuzzy category of red-ish colours like blood-red or rose-red.
Even if there is some academic "these light wavelengths are the ones" definition of red that is arbitrary. Aliens might have drawn the lines elsewhere.
To pick an example closer to the morality case: Hunger could equally be called a subjective phenomenon. Aliens may not experience anything like hunger. A rare genetic condition might produce a human who experiences escalating hunger as an escalating degree of pleasure.
But as humans making our way through the real world, we are forced by physical and anatomical reality to see the causes and consequences of human hunger as objective truths.
You didn't consider what I wrote - colourblind people get through life without considering the colour of traffic lights at all. They have to. They are colour blind. The colour of a traffic light as a practical matter is a subjective fact. In fact, the practical colour of a traffic light is more subjective than the philosophical take because at least philosophers can be convinced with a wavelength argument.
I define hunger as the ideal signal that corresponds exactly to the extent of starvation. 'Ideal signal' is almost like sensation or feeling, except it may not be always or in full precision accessible by consciousness. It is a physical signal, or a function from the physical state. Extent of starvation seems objective (after fixing its definition), and so hunger seems objective too (correspondence is exact). By this definition, if you are starved, but don't notice a feeling of hunger, it's that, you still are hungry, just don't notice it.
> What I consider hunger someone else can consider the norm or vice versa.
How can that be? Do you mean definition, or something else? What do you consider hunger?
If you redefine words how you please you can prove anything. What's the point tho?
> What do you consider hunger?
The feeling I sometimes get that I should eat something. It's VERY subjective (for example when I'm fasting I can go a few days without it, but when I'm not - I get it just cause I'm bored and haven't eaten a breakfast).
Otherwise, this question comes down to whether the meaning of the term 'red' is generated objectively or subjectively, which is related to philosophy of language and mind. See article: https://en.wikipedia.org/wiki/Linguistic_relativity_and_the_... .