I think it looks fine as it is from the pics you posted, personally I wouldn't worry about it.

So Caveat: I don't have much experience with paint and like you have no idea how to do airbrushing.

However that said if you're sure you want to try painting the scales I suspect airbrushing would be the most effective, if you can't though I have a few ideas and a few things to keep in mind.
Ideas (some of them might be a long shot so test on extra scales first):
- Small brush with thinned down paint
- Tinting some type of glue/adhesive instead of paint
- Dye maybe... Not sure what type of plastic the scales are so might not work.
- Some plastics go yellow in the sun, you could test a few scales by leaving them on your windowsill.

Things to keep in mind:
- Most paints don't like to stick to plastics
- Any coating on the scales is likely to rub/flake off as the scales rub together while you swim
- Because of that I suspect public swimming pools won't like/let you use your tail there
- If you even think you might swim in natural water sources (rivers, lakes, sea, etc) you need to make sure that whatever you coat the scales with either can not come off at all (and remember that ANY coating is likely to rub of just from movement) or is safe for marine/aquatic life, both flakes that might rub off and anything that might leech out or dissolve in water.
- It's going to be a massive job to do by hand since you can't airbrush, perhaps see if there is someone near you willing to teach you, or a hobby course or something, cause I suspect airbrushing will be the best option.

Happy to be corrected if I've posted something incorrect, like I said, I don't have much experience with paint.

Good luck, whatever you decide to do though and great job on the tail already, it looks awesome