Change the color of the field.

Override:

• GUIColor(EColor eColor, float alpha = 1f)

  Use EColor with custom alpha value

• GUIColor(string hexColorOrCallback)

  Use hex color which starts with #, or use a callback, to get the color

• GUIColor(float r, float g, float b, float a = 1f)

  Use rgb/rgba color (0-1 range)

// EColor + alpha
[GUIColor(EColor.Cyan, 0.9f)] public int intField;
// Hex color
[GUIColor("#FFC0CB")] public string[] stringArray;
// rgb/rgba
[GUIColor(112 / 255f, 181 / 255f, 131 / 255f)]
public GameObject lightGreen;
[GUIColor(0, 136 / 255f, 247 / 255f, 0.3f)]
public Transform transparentBlue;

// Dynamic color of field
[GUIColor("
quot; + nameof(dynamicColor)), Range(0, 10)] public int rangeField;
public Color dynamicColor;

// Dynamic color of callback. `


















 can be omitted
[GUIColor(nameof(DynamicColorFunc)), TextArea] public string textArea;
private Color DynamicColorFunc() => dynamicColor;

// validation
[GUIColor("
quot; + nameof(ValidateColor))]
public int validate;

private Color ValidateColor()
{
    const float c = 207 / 255f;
    return validate < 5 ? Color.red : new Color(c, c, c);
}

It works with ShowInInspector, Button, InfoBox, etc. too:

[GUIColor(EColor.Gold)]
[InfoBox("This is colored gold using <b><u><color=white>GUIColor</color></u></b> attribute.")]
[Button]
private void ButtonGold(){}

[ShowInInspector]
[GUIColor("#00FF00")]
private int _greenInt = 42;

[ShowInInspector]
[GUIColor(EColor.Burlywood)]
private int Calc([PropRange(0,  10)] int v) => v + Random.Range(1, 9) * 100;

[ShowInInspector]
[GUIColor("
quot; + nameof(GetColor))]
[InfoBox("Dynamic callback")]
private DateTime _dt;

private byte _color255;

private Color GetColor()
{
    _color255 = (byte)((_color255 + 10) % 256);
    byte r = (byte)(_color255 * 1 % 256);
    byte g = (byte)((_color255 * 2 + 100) % 256);
    byte b = (byte)((_color255 * 3 + 100) % 256);
    return new Color32(r, g, b, 255);
}

(UI Toolkit implementation code is partly from EditorAttributes, go give a star to them!)