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I would have thought in a Sci-Fi world you could tint whatever colour you fancy. I'll say this though, green makes perfect sense as it's a tint commonly used by the military to filter direct overhead light such as the perspex panels atop helicopter cockpits. Those are typically tinted green. Reduces glare and UV exposure.
I'm sure clear blue works just as well but I've never had any problems using any of the readily available clear green paints out there, be that Vallejo, Gunze Sangyo or Tamiya. The viscosity is usually good to go straight out of the bottle/jar and provides good good streak free coverage if you are careful. As ever, test piece first and you can always add a little water or flow improver if you want better coverage.
The HB pencil is a handy tool if you are looking at making scratch marks and nicks to paint. If you want older, more realistic scratches, apply either a lightened version of the surrounding paint and/ or what the undercoat should look like before applying the scratch mark either with a brush or pencil. This way it looks as of the paint has flaked a little as a result of the scratch.
Less is always more with distressing. Too often we see kit that looks like it's been sitting at a car wrecking yard for years. Militaries are big on maintenance, including painting but that's also true of other organisations. Tiny dabs with a fine brush are best, although there are advocates of foam and for heavy areas, shortened bristles like on an old brush you woud use for dry brushing. You can apply by lightly jabbing teh surface with an almost dry brush.
Final quick tip. Gundam panel marking pens, especially the grey ones, are a really quick and easy way to add depth by darkening the er... panel lines. Most model and hobby shops stock them these days.