Extension of a TextMeshProUGUI Editor that includes a workaround to fix wrong outline color. Place this file in the Editor folder.

FixedTMPColorsEditor.cs

using TMPro;
using TMPro.EditorUtilities;
using UnityEditor;
using UnityEngine;

namespace LukaszZmudziak
{
    [CustomEditor(typeof(TextMeshProUGUI), true), CanEditMultipleObjects]
    public class FixedTMPColorsEditor : TMP_EditorPanelUI
    {
        private static bool _customToolsFoldout = false;

        private static readonly GUIContent FixedFaceLabel = new GUIContent("Fixed face color");
        private static readonly GUIContent FixedOutlineLabel = new GUIContent("Fixed outline color");
        private static readonly GUIContent FixedUnderlayLabel = new GUIContent("Fixed underline color");
        
        private Color _outlineColor;
        private Color _underlayColor;
        private Color _faceColor;

        protected override void OnEnable()
        {
            base.OnEnable();

            _faceColor = ReverseTMPOutlineColor(m_TextComponent.faceColor);
            _outlineColor = ReverseTMPOutlineColor(m_TextComponent.outlineColor);
            _underlayColor = ReverseTMPOutlineColor(m_TextComponent.fontSharedMaterial.GetColor(ShaderUtilities.ID_UnderlayColor));
        }
        
        public override void OnInspectorGUI()
        {
            base.OnInspectorGUI();

            DrawCustomTools();
        }

        protected void DrawCustomTools()
        {
            var rect = EditorGUILayout.GetControlRect(false, 24);

            if (GUI.Button(rect, new GUIContent("<b>Custom tools</b>"), TMP_UIStyleManager.sectionHeader))
                _customToolsFoldout = !_customToolsFoldout;

            GUI.Label(rect, _customToolsFoldout ? k_UiStateLabel[0] : k_UiStateLabel[1], TMP_UIStyleManager.rightLabel);
            
            if (_customToolsFoldout)
            {
                DrawColorField(FixedFaceLabel, ref _faceColor, ShaderUtilities.ID_FaceColor);
                
                DrawColorField(FixedOutlineLabel, ref _outlineColor, ShaderUtilities.ID_OutlineColor);
                
                DrawColorField(FixedUnderlayLabel, ref _underlayColor, ShaderUtilities.ID_UnderlayColor);
            }
        }

        public void DrawColorField(GUIContent label, ref Color color, int shaderPropertyID)
        {
            var newColor = EditorGUILayout.ColorField(label, color, true, true, true);
            if (color != newColor)
            {
                color = newColor;
                    
                m_TextComponent.fontSharedMaterial.SetColor(shaderPropertyID, FixTMPOutlineColor(newColor));
            }
        }

        private Color FixTMPOutlineColor(Color input) 
            => new Color(Mathf.Pow(input.r, 0.4545f), Mathf.Pow(input.g, 0.4545f), Mathf.Pow(input.b, 0.4545f));

        private Color ReverseTMPOutlineColor(Color input) 
            => new Color(Mathf.Pow(input.r, (1 / 0.4545f)), Mathf.Pow(input.g, (1 / 0.4545f)), Mathf.Pow(input.b, (1 / 0.4545f)));
    }
}