Confusion about the construction of the rat's mental map
You have a couple small misunderstandings that I think are making it hard to understand the figure. There is no map of the hippocampus pictured here!

1) Note: this is a schematic figure. These aren't real data, although there are real data that show this pattern. This is meant to be an easy to understand diagram that explains how place cells work.

2) Panel (a) is EXTERNAL SPACE. Essentially it's a "top down" view of a floor that the rat is running around on.

3) Panel (b) shows spike trains for three different cells: theses cells have been arbitrarily color-coded. The hash shows the times that cells 1, 2, or 3 fired spikes. There are just three cells shown in this figure: there aren't any combinations of cells considered one color or multiple cells in one color, just three cells and three colors.

4) Panel (a) is showing how researchers have connected spiking data, like in (b), to the animal's location. The procedure is this:


Start with a blank map of space.
Record from Cell 1. Every time cell 1 fires, mark a blue dot in panel (a) indicating where the animal was when Cell 1 fired.
Record from Cell 2. Every time cell 2 fires, mark a green dot in panel (a).
Record from Cell 3. Every time cell 3 fires, mark a red dot in panel (a).
(the previous steps could be done for many more than 3 cells simultaneously or in sequence; in this example they are just using 3 cells)


5) Panel (c) is in world space, just like in panel (a), and it shows a hypothesized way that the animal could be using the activity of cells 1, 2, and 3 to determine where it is in space. A researcher recording from cells 1, 2, and 3 could also use those recordings to make a guess of where the animal is. If only cell 3 fires within some window of time (an appropriate window is indicated by the outlined ovals in (b) ), and cells 1 and 2 do not, then the best guess is that the animal is in the upper left area that's shaded only red. If all three cells fire, then the best guess is that the animal is in the very center overlapped region of space.

The dashed line shows a hypothetical path the animal might take through it's environment. If the animal took this path, we'd expect to see place cells firing just like they did in panel (b): first only the blue cell, then blue+green, then all three, then red+green, and finally red only.

6) Very importantly: the animal has way way more cells encoding place than just these three. It would be really confusing to plot them all out in discriminable colors like these, but it is this more complex map with thousands of dimensions that gives the animal a good internal representation of its current location.

Hope this helps!