Our brains are very good at differentiating between different energy levels (frequencies) of light. This skill is way more important for organisms that can actually directly harvest that energy using pigments, like plants and bacteria, but we've evolutionarily figured out how to put it to good use as a way to locate our prey.

Probably slightly different. There are 3 different types of cones in the eye that respond to the primary colors differently, and since everyone is different the ratios between the numbers of the cones will be different. However, the software our brains develope to interpret the raw data coming from our eyes may compensate to some degree.

It's impossible to compare subjective perceptions, but I tend to think they are largely the same. The programs that are passed down generations are consistent. My yellow is my mom's yellow, Occam's Razor.

I have always thought that this could be so. I will add that I think there must be some restrictions on our perception - for example I might consider that what I see as green (writing, say) on a red background is a jarring experience and (I think) it is likely that you will also consider it's 'painful on the eyes'. So in this example, although 'my' red may seem different to me from how 'your' red seems to you we still agree on the effect of the combination of these colours together. This holds even though we agree that grass (for example) is a shade of green and not yellow or blue. I hope these thoughts make some sense. On a vaguely related note have a look at Le Courier's legibility table. That might help.