The Powder Toy

That's already been done. It's cracker's paste/colloid element.    It's not technically a viscous fluid, though. It's just been modified to turn solid anywhere over...

Something that has the properties of liquid but without the intense water-spreading behavior would be fun too.

It doesn't.      Basically pressure is arbitrary for all we can tell. I can ask Skylark next time he's around.

navman  In Cracker's mod.    ...You know, check the mods category on the forums. : P

Honestly there's no simple way to tell whether an idea would be easy or difficult to implement.    Considering some ideas that I thought would be hella difficult to make turned out to ...

Flarn, use activated liquid crystal. :P It meets every requirement you described in your post and then some.    It doesn't reflect, bend, or otherwise color photons.

I agree a tiny bit about the green. Would probably be easy-ish to fix.    Also, as far as materials that are transparent to photons but do not refract them:  https://powdertoy.co.uk/For...

You could just carefully experiment with bouncing photons off of colored substances to achieve the colors you want.      ...Or use heated Glow to achieve a similar but more expensive e...

Would it be possible for these forums to display whether a user has read a particular thread's most recent post, whether all the threads in a particular category have similarily been read? ...

You mean like Ms Paint's line curling button? :D