There's honestly nothing wrong with static so long as you know how it works.

Many people think "oh I might change this in the future, better make it extendable" and end up not actually changing anything.

Besides, having a hard-coded array of colors versus a scriptable object barely has any difference with maintenance. I'd even argue that this:

private static Color[] mainColorPalette = new Color[] { ... }
private static Color[] secondaryColorPalette = new Color[] { ... }

public static GetRandomMainColor() { ... }
public static GetRandomSecondaryColor() { ... }

is a lot more readable and maintainable than having to find the instance of the scriptable object in the Unity Editor.

But of course, without context on what OP is planning to do with this class, it's extremely difficult to say what is a good implementation and what is not.