OK, let's look at a small portion:
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You're quite right that colours come from thin-film effects: in some processes poly is red and active is green, in some processes it's the other way around.
If we start by recognising the typical shapes of poly and active (diffusion) where they are not covered by metal, it will be easier to later identify them where they are covered.
Poly is generally thin, whereas active is usually larger: poly is going from one place to another whereas active is covering an area. The crucial thing is that poly crosses active to make a transistor: so it overlaps an active region a bit and then stops. Both poly and active will surround any contacts - so spotting the contacts first will help. If there's ambiguity about where the edge of active is, looking at the poly will often help.
In this case poly is red whereas active is blueish. The field oxide (the negation of active) is greenish. There's just one layer of metal, and therefore only one type of contact cut which connects metal down to whatever is below. This process also allows for a butted or buried contact, where poly connects to active (without making a transistor)
I've traced the edges of some active here, and the centrelines of some poly - there's are two mirrored versions of the same structure which should help you see which edges I've followed:
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As every contact cut must connect from metal down to something, you can use that to orientate yourself as to whether you're inside a shape or outside it.
The L-shaped fat poly towards the right is indicating a weak transistor: normally that will prove to be a pullup. It's still poly crossing active, but it looks like a thin meander of active running under a big rectangle of poly.
The fat poly to the left (inverted L shape) is certainly a pull up. In this case at the north edge where the active approaches the poly you see a faintly outlined rectangle. That's the buried contact: the poly and active are electrically connected. That's typical for the output of a gate: the poly continues south as a normal width line, carrying that output signal.
Once you're under metal, you need to look at edges and see which shape and layer they must logically belong to.
Hope that helps a bit: I feel I could have done better...
Cheers
Ed