i need some good plant sets for my project, mainly bright colours like purples, oranges, blues, ect... i came up with this with the plant sets i have but it doesnt quite look right

If it comes to Sci-Fi foliage in RCT3 we run out of CSO's if i remember it correctly.
Because most of the Com builds a realistic way, so there wasn't really a need for something like that, so no one creates that and/or released it.
What you can do is that you use trees that have special colors like asian cherry trees that have light purple leaves or normal trees that have autumn looking ones.

But the alien look will be still missing.

Another option would be to go for more city/base style on an astroid/meteor. So no foliage is needed and you can use rocks like crazy.
Alternative if you want foliage, build it like a future looking moonbase, where plants are under a giant glasdome, like "hydro cultures". With a big future greenhouse you can use normal looking plants in a space themed area without looking that strange

But specific cso sets that are sci-fi and have plants/trees in it, i think the answer is no.

So long, Cobra