It's not that you physically can't see it. Rather, you don't have a mental distinction for it. Two different greens are more or less the same color. So there's no real need to distinguish. If you have words for two different greens, you'll distinguish between them. Green and "Forest Green" are clearly two different colors. But if you never used forest green, you'd simply call it green, and not make a fuss about the difference.
The japanese can clearly see green/blue. But they call them both 'green'. As such, in early anime/manga, you can see streetlights drawn as blue, even though they call it green (and the japanese streetlights IRL are green).
Similarly, this distinction business doesn't just go for colors, but everything. You don't distinguish between a Macbook Air and Macbook Pro, they are just "a mac". Because you don't have a need to distinguish them.
You don't distinguish between different distros of linux, they are just 'linux'.
You don't distinguish between different levels of consciousness, they are just 'consciousness'. There's certainly different levels of consciousness/awareness. As any lucid dreamer can tell you. There's 'not present, but remembered', 'present, but didn't remember', 'present and remember (lucid)', and 'not present, didn't remember'. There's almost certainly 'levels' between those. But most people don't care to distinguish. And most people don't even realize there's a difference between being lucid and not being lucid.
Is there a need to distinguish between british and american english? For some, yes. I use the spelling interchangeably because there's no real need to distinguish between the two. They are both 'english'.