Fealist you should be able to enter any valid CSS color value in there, so probably yes. A linear-gradient() filter should work.
Some fields might have validation rules or max length that prohibits entering the value, but you could go around the validation by entering the value directly in the database.
If the value doesn't render, you can check with the browser inspect tool whether the value was inserted correctly. The value is inserted as the color or background CSS property by Mithril, so maybe not all values can be set that way. But gradients probably can. It's basically the same as doing document.getElement(...).style.color = 'value', so any value valid for javascript style manipulation should work.
If that doesn't work you can also use custom CSS and target the elements directly. For tags I think most tag labels should have the tag ID or slug added to it, which you can use inside of the custom CSS.
One benefit of using custom CSS for this is that you can define a fallback color as well.
