I use this ND big stopper filter, which was recommended to me by my photographer friend. It works great and for the price, I can't complain. It's a screw on one.
https://www.amazon.co.uk/Neutral-Den...ywords=ND+1000
I have these cheaper grad filter also. They're OK, but they do scratch and get dirty quite easily. I hope to eventually buy some high quality Lee filters further down the line, but to start off with, they're not too bad.
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
The filters do give colour cast but that is normal. Some of the high end ones have reduced colour cast but then obviously you're paying big money for them. If you shoot in RAW, it's not so much an issue anyway as you can correct this during post production.