And how about the color blue? What if the way you see blue is orange, and the way you see green is black? If I look through your eyes I will see an orange sky and black grass, which you have learned your whole life as green and blue?
To be fair, that's answerable scientifically, if you're prepared to accept that the words we use for different colours are essentially arbitrary and had history gone a different way we'd be calling them something completely different. Indeed, they already are - in different languages.
The scientific part behind colour is that all colours we see are influenced by whatever part of the spectral wavelength is being picked up by our eyes.
Light is made up of various wavelengths, and those in the visible spectrum have different colours. Even if nobody on Earth could see in true colour, red would still be red, blue would still be blue because that's what light wavelength is being reflected.
Infra red and ultraviolet are good examples of this. You can't see either colour, so how do we know that infra red is really red wavelength light (but longer), and ultraviolet really violet wavelength light (but shorter)? Mainly because each has a correspondingly different wavelength over and under visual light (to humans) in the spectrum.
Going back to your original point, since we can measure the wavelength of different colours of light and two people on either side of the world can agree on a particular colour (provided they aren't colour-blind), we can define colours with reasonable certainty. It's then simply up to what you fancy naming them, but our language has settled on names now so we're good
As far as round/donut-shaped goes, similar deal. If we called spheres donuts and donuts spheres, that'd simply be a different naming norm, rather than something up for philosophical debate. Since they're two distinctly different shapes, it doesn't really matter what we call them as long as they both aren't called the same thing.
The only issue there would be if the shape was such that when viewed and measured from different points, it looked like different shapes, and remained different to different people, wherever they observed the shape from. To all intents and purposes though this is impossible, with the potential exception of "shapes" at a quantum level...
But back on topic, to what we can observe in our consensual reality, it is safe to say that the earth is almost definitely not flat.
What do you mean
almost definitely?
It's entirely safe to say
with certainty that the Earth
isn't flat, because there are a thousand ways of proving it and zero ways of disproving it. Since the only way we can observe is with our own observations, all other opinions on the Earth's shape are null and void.
Seriously, quit with the philosophical BS in scientific discussions. It's irritating.