Thanks for your questions but (everyone) please ask them one or two at a time. When I get presented with a wall of questions I sometimes never find time to answer at all.
1) Highlight (specular reflection of source)
"In general, reflected light is diffusely reflected, i.e. scattered, in all various directions if the surface is not perfectly smooth."
This explanation of diffuse reflection is often encountered but is completely wrong. The diffuse reflection (red colour) of an apple is the result of subsurface scattering, not surface roughness. Specular and diffuse reflection both occur over the whole surface of the apple; the highlights are just the most conspicuous specular reflections and occur where the surface is at just the right angle to bounce light from a light source to your eye.
"A more saturated color appears "brighter" than a less saturated color, correct? Why?"
A saturated object colour by definition is relatively intensely coloured compared to an unsaturated object colour of the same greyscale value. However it's true that say a cadmium scarlet might have the same greyscale value as a middle grey (so that the two blend when you squint) and yet give a greater impression of brightness to most people. This impression of brightness has been called "brilliance" and I think it results from the fact that the cadmium scarlet is at the maximum brightness possible for its hue and saturation while the middle grey is not.
"Are we talking about being really identical perceptions of just very similar?"
"Metamerism, based on my understanding, consists in the fact that certain objects show a different perceived color under different illuminations (which have different emission spectra)."
Metamerism refers to the matching; when the match breaks down under a different illumination it is called metameric failure.
"Every object has a certain reflectance curve which together (multiplied?) with the spectrum of the source (and the eye) determine the "color" of the object. How do certain objects manage to maintain the same color under different illuminations?"
The colour of the light reflected by an object does change when the colour of the illumination changes, but we are usually pretty good at automatically discounting these effects of lighting. This ability of our visual system is called colour constancy. So unless conditons are particularly difficult (e.g. monochromatic lighting or limited visual cues) the perceived colour of an object depends mainly on its reflectance curve.
"At what point does a white start becoming a gray? What is the intensity threshold to perceive gray? What about gray and black?"
What we call white and black depends on what else is in the visual field. For example a Munsell value 9 chip, which reflects about 80% of the light falling on it, looks white if nothing brighter is nearby but light grey if placed beside something lighter.
"Why are there some many different types of whites each having different spectra?"
Because it's very easy for a light to appear white, it only has to produce an effectively equal response of our three cone types, so the spectrum can be spiky on a small scale in any way imaginable as long as the overall distribution is even. It's much harder for an object to be white; it has to reflect balanced white light and plenty of it, so it pretty much has to reflect all wavelengths at a high level.
"Brown is not a really new color. I think it is what we perceive orange at low luminosity."
"Do you know of a free internet applet that lets me control the proportion of RGB to see the resulting color?"
There are a few Java applets out there but I find it hard to get these to run now because of the security controls built into my browsers. There is an interactive flash demonstration on my website (Figure 4.2.1) that lets you do this by adjusting the sliders (won't work on android or i-crap of course).
Hope that helps! If you have any follow up questions I'd appreciate it if you'd ask them one at a time and wait for an answer before asking the next!