First of all, It is strongly recommended that you use the updated unity GUI tools. Check UI-system-compare.

Back to this question. Let's look at the definition of GUI-backgroundColor:

Global tinting color for all background elements rendered by the GUI.
This gets multiplied by color.

1. It's a global value for all elements.
2. It's not the element color but a "correction value". Just like "multiply blend mode" in photoshop.

Maybe this button Need to have its own GUIStyle object. Try this:

    var playButtonStyle = new GUIStyle(GUI.skin.box);
    if (test == false)
    {
        playButtonStyle.normal.background = MakeTex(2, 2, Color.red);
        test1 = "STOP";
    }
    else
    {
        playButtonStyle.normal.background = MakeTex(2, 2, Color.green);
        test1 = "PLAY";
    }
    if (GUI.Button(new Rect(10, 130, 170, 30), test1, playButtonStyle))
    {
        Debug.Log("Clicked the button with text");
        test = !test;
    }

First of all, It is strongly recommended that you use the updated unity GUI tools. Check UI-system-compare.

Back to this question. Let's look at the definition of GUI-backgroundColor:

Global tinting color for all background elements rendered by the GUI.
This gets multiplied by color.

1. It's a global value for all elements.
2. It's not the element color but a "correction value". Just like "multiply blend mode" in photoshop.

Maybe this button Need to have its own GUIStyle object. Try this:

    var playButtonStyle = new GUIStyle(GUI.skin.box);
    if (test == false)
    {
        playButtonStyle.normal.background = MakeTex(2, 2, Color.red);
        test1 = "STOP";
    }
    else
    {
        playButtonStyle.normal.background = MakeTex(2, 2, Color.green);
        test1 = "PLAY";
    }
    if (GUI.Button(new Rect(10, 130, 170, 30), test1, playButtonStyle))
    {
        Debug.Log("Clicked the button with text");
        test = !test;
    }

Another way:

    var defaultColor = GUI.color;
    if (test == false)
    {
        GUI.color = Color.red;
        test1 = "STOP";
    }
    else
    {
        GUI.color = Color.green;
        test1 = "PLAY";
    }
    if (GUI.Button(new Rect(10, 130, 170, 30), test1))
    {
        Debug.Log("Clicked the button with text");
        test = !test;
    }
    GUI.color = defaultColor;

Modify the global color of GUI and restore it after use. This preserves the style of the default button.