I think you understand the experiment and your results pretty well now even though what's going on is a bit complicated. Here are some specific comments on your last post.
You ask, "If assuming the color density are the same, how can you know some colors are darker than the others?" The color density and the degree of darkness are not separate indicators of a color. For example, if a red color were actually a blend of red and white, it would give a pinkish appearance and would reflect not only the red light hitting it, but also some of the other colors. If a red were very pure red, it still might reflect only a narrow range of wavelengths centered on red (650 nm), or it might reflect a wider range of wavelengths centered on the same wavelength. The paper that reflected the wider range of wavelengths would appear brighter (less dark) but both would be relatively pure red. You would need to know the reflectance of each of your papers at all wavelengths to predict just how much light they absorb and convert to heat. Since you don't have this information, let's just assume each paper reflects a pure color band and each has the same wavelength range.
Yes, you are right that black (pure black) absorbs all colors, and white reflects them all. That is why you got black at the most-heated end of the list and white at the least heated end.
Blue comes next in your list because there is relatively little blue even in sunlight and it absorbs everything else. Sunlight has its peak energy in green, so why doesn't green appear closer to white? Yellow and orange are different densities of the same wavelength. Orange can be produced by either light of about 570 nm, or it can be produced by an equal combination of red and green. Yellow is just a brighter (more reflective) orange. It's probably a good guess to say that your yellow and orange papers have a relatively wide reflection band. The heating power of red falls in between orange and yellow because it reflects green light whereas yellow reflects red and green.
The only way I see to explain why green is not cooler than red is to assume that the green paper reflects not all of the green light. The paper maker likely decided that pure green looked too light to match the other papers. Another possibility is that your sunlight was filtered by smog or some other condition that reduced the amount of green relative to red.
You can get a feeling for how much of individual colors make up a particular appearance by experimenting with color mixing on your computer. If you have a word-processing or drawing program, you can use the color tool to see how much red, blue, and green it takes to make a color, or how to set hue, saturation, and luminance to get the result. If you compare colors you get on your screen with the papers you have, you might (might, not will) get a rough idea of the absorption and reflection of each paper. For example, you will see that full-on red and green produce yellow while partial but equal amounts of red and green produce orange.
Hope this helps a bit more, WW