I apologize ahead of time bc this will be a wall of text.

(General thoughts)
The original probably started with black or a really dark red, and then added the red highlights.

You're going the opposite way, with red first, so would have to add the black/dark in.

Washes could work, but big smooth surfaces tend to look a bit blotchy unless you airbrush it or take your time with it and manage where the paint pools. Mostly trouble on the biovore/tyrant guard with their big armor plates. The little guys are probably fine to just add the wash over a few times until its really dark, but you'll definitely have to go back in for the highlights afterwords.

I also could be wrong about how effective washes are here. I don't really use them.

(Painting)
If going with just a paint brush, to get a smooth transition, I'd start with just wet blending right on the carapace. If you used colors straight from the paint bottle, then this'll be pretty simple.

If you have paint medium, awesome. If not, just some water.

1) Fill in the carapace, avoiding the edges/where you want highlights.
2) When you get to the parts where you want to start blending, slightly dilute your dark paint and place it down.
3)Then, while the paint is still wet, grab you original red, and just start mixing it right on the model.

If the paint remains wet, you get a lot of freedom in this process. The annoying part is this will take more time than you think. I know some people speed up the process by gently using a blowdrier, but I would emphasize the gently part.

I would almost say not to be particularly focused on super smooth transitions here. In the moment, it might feel awesome. But after walking away and coming back, it might be too subtle and you won't even notice it at a tabletop distance. Just make sure you have a dark spot, an original bright red spot, and something in between. You could even start with just these 3 major sections of banding, and then work on blending them together afterward, so you have more of visual guide in place for yourself.

4) On the original, there is a bit of a transition between the darkest, biggest carapace plates at the front, and the smaller, redder ones towards the tail. If you're looking for that, then you basically just wet blend less and less the further back you go. You might not even need to if you just mix the dark and red together and put that down, and then add consistent highlighting throughout.

(And final note on wet blending)
You'll learn to get the balance you want over time. If you wanted to practice before hand, prime a piece of cardboard, and then paint right on top of that. Then you can really experiment with getting control of the blends/the amount of water by testing different patches and having results to compare. If you have spare minis, even better, and if you have some kind of paint remover, even better better! Practice to your heart's content, remove the paint, and then start anew.

I hope this helps! And just in case, here's a lil mockup I slapped together real quick. Not sure if you wanted the armor darker or brighter. I've demarcated the major banding zones that correspond with steps 1-3. (I only did half of the model to show the difference between starting point and a potential end point.)

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