diff --git a/color.py b/color.py
index 03fb655..4068634 100644
--- a/color.py
+++ b/color.py
@@ -84,29 +84,39 @@
 
 DEFAULT = None
 
+# Placholder value that indicates we need to check if the user is in an
+# interactive terminal session to determine if we turn on color or not.
+_CHECK_CONSOLE = object()
+
+# https://git-scm.com/docs/git-config#Documentation/git-config.txt-colorui
+_CONFIG_TO_COLOR_SETTING = {
+    "false": False,
+    "never": False,
+    "no": False,
+    "auto": _CHECK_CONSOLE,
+    "true": _CHECK_CONSOLE,
+    "yes": _CHECK_CONSOLE,
+    "always": True,
+}
+
 
 def SetDefaultColoring(state: Optional[str]) -> None:
     """Set coloring behavior to |state|.
 
     This is useful for overriding config options via the command line.
     """
-    if state is None:
-        # Leave it alone -- return quick!
-        return
 
     global DEFAULT
-    state = state.lower()
-    if state in ("auto",):
+
+    if isinstance(state, str):
+        state = state.lower()
+    if state in _CONFIG_TO_COLOR_SETTING:
         DEFAULT = state
-    elif state in ("always", "yes", "true"):
-        DEFAULT = "always"
-    elif state in ("never", "no", "false"):
-        DEFAULT = "never"
 
 
 class Coloring:
     def __init__(self, config, section_type):
-        self._section = "color.%s" % section_type
+        self._section = f"color.{section_type}"
         self._config = config
         self._out = sys.stdout
 
@@ -115,16 +125,12 @@
             on = self._config.GetString(self._section)
             if on is None:
                 on = self._config.GetString("color.ui")
+        if isinstance(on, str):
+            on = on.lower()
 
-        if on == "auto":
-            if pager.active or os.isatty(1):
-                self._on = True
-            else:
-                self._on = False
-        elif on in ("true", "always"):
-            self._on = True
-        else:
-            self._on = False
+        self._on = _CONFIG_TO_COLOR_SETTING.get(on, _CHECK_CONSOLE)
+        if self._on is _CHECK_CONSOLE:
+            self._on = pager.active or os.isatty(1)
 
     def redirect(self, out):
         self._out = out
diff --git a/tests/test_color.py b/tests/test_color.py
index 8b75d21..72d7499 100644
--- a/tests/test_color.py
+++ b/tests/test_color.py
@@ -14,6 +14,8 @@
 
 """Unittests for the color.py module."""
 
+from unittest import mock
+
 import pytest
 import utils_for_test
 
@@ -24,9 +26,14 @@
 @pytest.fixture
 def coloring() -> color.Coloring:
     """Create a Coloring object for testing."""
+    return _make_coloring("always")
+
+
+def _make_coloring(default_state: str) -> color.Coloring:
+    """Set the default color mode and return a Coloring using test config."""
     config_fixture = utils_for_test.FIXTURES_DIR / "test.gitconfig"
     config = git_config.GitConfig(config_fixture)
-    color.SetDefaultColoring("true")
+    color.SetDefaultColoring(default_state)
     return color.Coloring(config, "status")
 
 
@@ -72,3 +79,76 @@
     assert val == "\033[2;34;47m"
     val = coloring._parse("empty", "green", "white", "bold")
     assert val == "\033[1;32;47m"
+
+
+class TestSetDefaultColoring:
+    """Tests for SetDefaultColoring."""
+
+    def test_none_leaves_default_unchanged(self) -> None:
+        color.DEFAULT = "auto"
+        color.SetDefaultColoring(None)
+        assert color.DEFAULT == "auto"
+
+    @pytest.mark.parametrize(
+        "value, expected",
+        (
+            # auto/true/yes all store their lowercase form.
+            ("auto", "auto"),
+            ("Auto", "auto"),
+            ("true", "true"),
+            ("True", "true"),
+            ("yes", "yes"),
+            ("Yes", "yes"),
+            # "always" stores as "always".
+            ("always", "always"),
+            ("Always", "always"),
+            # never/no/false store their lowercase form.
+            ("never", "never"),
+            ("no", "no"),
+            ("false", "false"),
+        ),
+    )
+    def test_maps_to_expected(self, value: str, expected: str) -> None:
+        color.SetDefaultColoring(value)
+        assert color.DEFAULT == expected
+
+    def test_unrecognised_leaves_default_unchanged(self) -> None:
+        color.DEFAULT = "auto"
+        color.SetDefaultColoring("garbage")
+        assert color.DEFAULT == "auto"
+
+
+class TestColoringInit:
+    """Tests for Coloring.__init__ color mode logic."""
+
+    @pytest.mark.parametrize(
+        "state, isatty, pager_active, expected",
+        (
+            # "always" enables color unconditionally.
+            ("always", False, False, True),
+            # "never" disables color unconditionally.
+            ("never", True, True, False),
+            # auto/true/yes enable color only on a TTY or active pager.
+            ("auto", True, False, True),
+            ("auto", False, False, False),
+            ("auto", False, True, True),
+            ("true", True, False, True),
+            ("true", False, False, False),
+            ("true", False, True, True),
+            ("yes", True, False, True),
+            ("yes", False, False, False),
+            ("yes", False, True, True),
+        ),
+    )
+    def test_color_mode(
+        self,
+        state: str,
+        isatty: bool,
+        pager_active: bool,
+        expected: bool,
+    ) -> None:
+        with mock.patch("os.isatty", return_value=isatty), mock.patch(
+            "pager.active", pager_active
+        ):
+            c = _make_coloring(state)
+        assert c.is_on is expected