ソースコード

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;

internal static class Example
{
    [MenuItem( "Example/Hoge" )]
    private static void Hoge()
    {
        var colorPresetLibrary = AssetDatabase.LoadAssetAtPath<Object>( "XXXX.colors" );
        var type               = colorPresetLibrary.GetType();
        var countMethod        = type.GetMethod( "Count", BindingFlags.Public | BindingFlags.Instance );
        var addMethod          = type.GetMethod( "Add", BindingFlags.Public | BindingFlags.Instance );
        var removeMethod       = type.GetMethod( "Remove", BindingFlags.Public | BindingFlags.Instance );

        // すべての要素を削除してから
        while ( 0 < ( int )countMethod!.Invoke( colorPresetLibrary, Array.Empty<object>() ) )
        {
            removeMethod!.Invoke( colorPresetLibrary, new object[] { 0 } );
        }

        // 任意の色情報を追加
        addMethod!.Invoke( colorPresetLibrary, new object[] { Color.red, "あか" } );

        EditorUtility.SetDirty( colorPresetLibrary );
        AssetDatabase.SaveAssetIfDirty( colorPresetLibrary );
    }
}