> If in a langauge there is one word for 2 different colors, speakers of it are unable to see the difference between the colors.

Perhaps you mean to say that speakers are unable to name the difference between the colours?

I can easily see differences between (for example) different shades of red. But I can't name them other than "shade of red".

I do happen to subscribe to the Sapir-Whorf hypothesis, in the sense that I think the language you think in constrains your thoughts - but I don't think it is strong enough to prevent you from being able to see different colours.