I think probably the 1x2 yeah. Looks to be present in almost 3800 sets in around 80 colors.
I never understood how to get the right number...
On the 3004 (normal 1x2 brick) page, they have
* a list of ~70 normal colors, ~20 trans-clear colors and ~10 metallic colors
* a list of "known colors" with ~50 normal colors, a single trans-clear and no metallic
* a list of colors for price guide with ~60 normal colors and ~10 trans-clear
I'm pretty sure the "3004" trans-clear listed on Bricklink shops are actually 3065 bricks (1x2 without the tube), not 3004.
When there's only a single shop selling a color, I also suspect it's just a shop owner failing to identify the proper color (Light lime for example?).
That is really interesting. Do you have a palette of all colours Lego already used?
Not yet... There's probably a bit more than 50 colors for 1x2 bricks, I only have about 40. I started this year, trying to grab the missing ones from my bricklink orders. I haven't yet done specific order for this (I'll probably do it sooner or later).
The tricky part is that with uncommon colors, you often don't receive what you've ordered...
That being said, I have virtually all the current ones, and a large set of discontinued ones. Since I build it mostly as a reference for MOCs and bricklink orders (and sorting), it does the job. I mean... I doubt I'll try to build something in Maersk Blue...