Tip: Why don't you start repeating colours towards the second side of your main post? The important thing is just that no 2 nearby types of liquids would have the same colour. That would make each separate idea more noticable. Also, wow, there's a ton more ideas now since the last time I checked. Some seem quite weird and questionable, but nonetheless interesting.
Well, you've already got sulphur listed, though you don't mention any properties of it. If I may, I'd like to recommend a few: Sulphur ash that collects on the ground behaves much like a solid in regards to light - it's opaque. You can sink into it. Sulphur ash is a very fine dust and shifts at a moment's notice, so you can wind up sinking in it. It's like quicksand (which might be another liquid idea in and of itself). Once you start sinking, it's very hard to get out, and struggling only makes it worse. If it occurs as weather (ash 'snow' or volcanic duststorms), it can render the air unbreathable without some form of protection (ever heard of black lung? On a volcanic world sulphur ash could inflict a similar condition, only worse because the sulphur can react with the water in your body to produce sulphuric acid.) Of course, these are all suggestions and ideas; you may use/ignore/selectively pick whichever you see fit.
If you are on a gas giant planet it should rain acid. Pockets of it can be found deep underground as well. Hmm... I could imagine small floating islands of lighter solids to make it in one piece through some time on a gas giant, but I have little to no confidence of witnessing anything that could be called "deep underground" on a gas planet. I find your other suggestions very cool though! And when it comes to gas giants I remember reading about some super hot planets that featured iron rain in their atmosphere! Imagine the implications of that!
I like the idea of sludge. Maybe it would be a byproduct of factories or mines and the planet slowly becomes polluted and toxic
I'd love going to a planet and experience the panic of noticing that the rain there is BLOODY ACID!!! Seriously, that would be fun!
Would it be possible to have some sort of pyroclastic flows in Starbound? Here is an edited Wikipedia definition: A pyroclastic flow, or Pyroclastic density current, is a fast-moving current of hot gas and rock, which reaches speeds moving away from a volcano of up to 450 mph. The gas can reach temperatures of about 1,830 °F. Pyroclastic flows normally hug the ground and travel downhill, Their speed depends upon the density of the current, the volcanic output rate, and the steepness of the slope. They are a common and devastating result of explosive volcanic eruptions. Think of it like a massive volcanic tidal wave.
When I get a chance to, I'll try to add that in. Allright, I added it. I trimmed it down a lot, as to not have an even bigger wall of text, but it's in there.
"Goo- I don't know what its used for, its just more viscous than water. Can be run on. Otherwise makes you sink if not moving. (Maybe stuff spawns from it.)" Could be a resource for building one of my idea, the Stasis Barrier/shield? http://community.playstarbound.com/index.php?threads/stasis-shield-barriers-and-fields-tech.16503/
All I want is a liquid that you can walk on. You're not slowed by it; you're not sped up by it; you can place blocks on it; you can drive on it; it's completely harmless if you're on top of it, but if this stuff gets poured on you, you're probably screwed; it just acts like firm ground in every way. Except for the fact that it flows. If you have enough of this stuff, you could have some really neat terraforming/smoothing operations, simply by pouring a few hundred cubic meters of this stuff into a valley. And, what if it rained this stuff? You will have quite the problem if it gets into your base. Also, it should be less dense than water, so if you have a lake, and you want a base there (Plus an ecological nightmare) pour a few hundred gallons of this stuff over the lake and volia, you now have a nice glassy-smooth place to build your base and race your rocket-mechs and whatnot. If there's not enough of it to completely cover whatever body of water you're placing it on, then it gloops up like RL tar and oil does, upon which you could build your base, and it will be dragged around by the currents. Or if you attach a motor to the floating island, you have an instant houseboat. Also, pumping it would make some interesting moats/walkways/elevators/trap doors/traps.