K terminology, like K2, has been obsolete for decades. Now K2 means the world's second highest mountain. Even Wratten numbers aren't used much anymore, since real Wratten gel filters have been functionally replaced by coated glass ones from various manufacturers with their own numbering systems, though Tiffen still adheres to Wratten equivalents.
With filters, you tend to get what you pay for. Just like lenses, it helps to have high quality glass and multi-coating. This not only reduces flare but makes the filter easier to clean and more resistant to condensation. There are several excellent brands; but the most bang for the buck seems to be HMC multicoated Hoyas. They offer a basic yellow, a light orange, and then a medium red, but skip over deep orange, which is one of my most useful flavors, and get from Heliopan instead. I also find a medium green like Hoya X1 quite useful. I rarely use yellow per se, but do have them if needed.
I don't know how "grade" talk factors into any of this. Filters darken the complementary hue across the color wheel from what they are,
and in theory, to the extent of their own density. But colors in nature often respond somewhat differently from what you might expect, so there is simply no substitute for experimenting and experience with your preferred film. I own a lot of filters in various sizes; but when out in the field, try to carry a minimum, typically just a 22 deep orange, a 25 medium red, and an X1 or 58 medium green.
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