I'm starting to plan ahead for a gunge session and was looking for advice from you messy folk. I'll probably go for Messy Supplies gunge again (tried the xtra thick MS Gunge and enjoyed the consistency) but . . . . which of the colours are the most opaque? Not interested in translucent stuff. From your own experience, which is the thickest / non-seethru?

I think the least see-through we ever used was the grey for the "Sludge Maid" set:

https://saturationhall.umd.net/download_info/the-rubber-sludge-maid

Though that was original Dunk Tank Dave gunge, before we started using MS.

But as long as properly mixed, the pink is also pretty solid, as is the fluorescent green and the cobalt blue. And we just use standard MS gunge, I'd expect the thicker ones to be even more opaque.

As above. Note that extra thick gunge is really just a larger measure of the same powder - obtaining heavier and more viscous (but not necessarily more opaque) results per 10 litres of water.

There is no particular colour that will universally create more opacity because it depends entirely on the nature of the pigment used in the colour.

Non toxic powder paint is generally added to colour gunge these days and the body given to these is often provided by chalk or clay powders like kaolin in white and charcoal in black. I've found that adding copious amounts of additional powder paint to the gunge will increase its opacity - or going further - adding potters' clays. Although they will dull the mixture it can be made absolutely opaque this way. Mixing will take a longer time - but I promise you it's worth it. There are some examples here and there in my showreel.

You can also make an opaque sludge with a mixture bentonite and paint.

Messypup is really cool working on a project where each color combines on itself

I'd suggest using any gunge, and just add more paint - powder paint may be best then you can cram more in without making your gunge cold. Then just keep mixing it in and sticking your hand in to test if it's opaque enough yet, and if not stir in some more.

Messysupplies sell multiple sizes of bags of just the colours, and you can get powder paint on amazon too but it tends to be tiny bags.

You can mix it thicker- producing a sticker gunge that'll pile up and form thicker layers over your body. (creates the effect of it being more opaque too)

MS gunge (extra thick) mixed with 6-8 litres of warm water produces a very thick heavy gunge and opaque gunge.
(When it's mixed this thick the glass effect/translucent gunge is visually stunning too)

The other option MS offers is the new(ish) 'Clayzee Colours' which being clay based (bentonite) produces a totally opaque gunge. The colours are even more vibrant than MS gunge and being clay based this stuff feels amazing too,
(you'll want a powered mixer or electric whisk for this stuff- as it's a little harder to mix)

We found that adding an additive with our HEC Gunge produced a more opaque foamy deluxe gunge. We have tried it with the Xantham we have for sale at the moment with good results as well. As others have said, adding more paint will help but it's a fine balance between being too much and staining people. We did a couple of dunk tank shoots where we put a lot of paint in and it was totally opaque but was difficult to shower the colour off the models.

Yeah . . . probably wanting to avoid any smurf-type incidents

We've been there done that and had models go home looking like a Smurf