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I looked down at my tissue and decided I want this watery camo look on my Wave Serpents and Hemlock. Just the yellow and black, not the red part. Any ideas on how would I go about doing this?

https://imgur.com/a/wiLWv

The wet tissue and its porosity is the main reason you get it looking like that. With a non-porous surface like a 40k model, it's hard to achieve.

My recommendation would be working simultaneously with two colours, splodging the paint on wet on wet, or alternatively to use sponge painting (on a dry base layer) to get a similar effect.

Others may have better suggestions.

The only way I can think of is by using an airbrush with inks.

You could try to paint a coat of acrylic medium + dry retarder first. Then, with a pipette, add a drop of highly concentrated pigment (like an ink) to the still-wet surface. The pigments should diffuse in the wet layer, and maybe produce something similar (although it might just create clouds).
That's very unpractical and might not work at all, but that's the only thing I could come up with…

Yes to what fresus said. Retarder is your friend here.

I would start with a white undercoat and cover it with a yellow glaze. Then I would apply the black splotches with highly thinned paint. Do this to rough out the areas that will be pure black, and touch up the parts you want to be opaque afterwards.

Once that's done, I would do white splotches where I want the pure yellow to go and glaze the yellow on top. That will give you good control over the intensity of the color and allow you to simulate that watery look without it becoming chaotic.

There are a number of splotches that are blended between the yellow and the black. I would do those on top of the pure yellow splotches one at a time with simple wet blending. With this kind of paint job, it will be easier to go from light to dark than the other way around.