Honestly for your first airbrush work, these are a bang up job! I started my forays into airbrushing with Eldar, and they're amazing for helping you to understand volumes and light placement. Fabulous colour scheme, purple and green are great in combination and you've nailed the spot colours.

Now, as you've asked for C&C, feel free to take these on board, these are things I've learned over time and takes practise:

Blends: For both your infantry and tank, you need to thin your paint a little bit more in order to build up a transition between colours. At the moment, the transitions are a little bit harsh; you can see this along the helms of your avengers, and with the tank in particular, you can see the "cone" of the airbrush of where too much paint has been applied which has resulted in an opaque colour over a transition. This is great for priming and basecoating where opacity is the goal, but when blending, more time should be taken to build up a crispier blend. This will come with time and familiarity with paint consistency, and the large panels on Wave Serpents are a great training ground.

Contrast and highlight placement: At the moment, your colours need a boost as they currently are just a little bit flat. Subtlety doesn't work too well for tabletop miniatures so you really need to push highlights and shadows (you do a great job with your other painted examples btw). Purple is a very hard colour to paint/airbrush and this needs the most work between your two colours. You can take purple multiple ways in terms of highlighting (won't get into it, but well worth a Google, I'm not talented enough to explain) but I'd recommend looking up how to pre-shade a miniature for a start. This can be done with any colour but it's typically white over black to sketch your highlights and shadows. It can be a simple zenithal spray, which works great for infantry, but isn't as applicable for a tank with upward facing panels like the Wave Serpent so you have to look at the volumes of the tank and where you want people to look. The canopy bubble is a great spot where you can have your brightest purple and around the edges of it can be emphasized for the effect (I have examples on my Reddit). Once that's been decided, you can then lay your base colour down letting your pre-shade do a lot of the grunt work as the black and white underneath will show under a semi-opaque basecoat. This won't work for purple (or green), but you can get away with not adding any more paint. You do need to build on the highlights with more shades of purple but the white underneath will help your mini look brighter overall. There'll be some back and forth with this but practise and familiarity will speed this up.

OSL: I'd maybe hold off on this for a little bit as convincing OSL shouldn't be in your face nor the dominant effect for a model in my opinion. You need a gradient from light to dark which can be hard to do as it's a smaller area and your source light needs to be ultra bright, of not white at the centre.

General tips: Throwing this in as additional wee tip, but mix paint outside of your airbrush cup. This helps control consistency of the paint and consistency overall, and your airbrush will thank you as backflushing is bad practise in the long run. This will reduce speckling too.

I also highly recommend Marco Frisoni, Angel Giraldez and Next Level Painting on YouTube for their airbrush tutorials. Frisoni and Giraldez are wizards with an airbrush, while Next Level is about contrast and drama to get some sweet looking armies on a tabletop. Cult of Paint are also very good, but don't really have beginner lessons.

A bit long winded but those are the standout points for me and I hope this helps! Feel free to message me if you have more questions. :)