[2.x] refactor: Extract color-default logic as a pure function (#8817)

**Problem**
The color-detection logic in Terminal.scala is interleaved with side
effects (system property reads, environment checks) making it hard to
understand and impossible to unit test.

**Solution**
Extract a pure `isColorDefault` function that takes all inputs as
parameters and returns whether color should be enabled. The existing
`useColorDefault` delegates to it. Add unit tests covering all priority
levels and heuristic branches.

Fixes #6050

internal/util-logging/src/main/scala/sbt/internal/util/Terminal.scala

@@ -378,17 +378,41 @@ object Terminal {
       }
     }
   }
-  private def useColorDefault: Boolean = {
-    // This approximates that both stdin and stdio are connected,
-    // so by default color will be turned off for pipes and redirects.
-    props
-      .map(_.color)
-      .orElse(isColorEnabledProp)
+
+  /**
+   * Pure function that determines whether color output should be enabled.
+   *
+   * Priority (highest to lowest):
+   *   1. propsColor — explicit color preference from terminal properties
+   *   2. colorProp — explicit color option
+   *   3. logFormatOpt — log format option (None defaults to true)
+   *   4. terminal heuristic — console type, CI, and Emacs detection
+   */
+  private[sbt] def isColorDefault(
+      propsColor: Option[Boolean],
+      colorProp: Option[Boolean],
+      logFormatOpt: Option[Boolean],
+      hasConsole: Boolean,
+      isDumbTerminal: Boolean,
+      isCI: Boolean,
+      isEmacs: Boolean,
+  ): Boolean =
+    propsColor
+      .orElse(colorProp)
       .getOrElse(
-        logFormatEnabled
-          .getOrElse(true) && ((hasConsole && !isDumbTerminal) || isCI || Util.isEmacs)
+        logFormatOpt.getOrElse(true) && ((hasConsole && !isDumbTerminal) || isCI || isEmacs)
       )
-  }
+
+  private def useColorDefault: Boolean =
+    isColorDefault(
+      propsColor = props.map(_.color),
+      colorProp = isColorEnabledProp,
+      logFormatOpt = logFormatEnabled,
+      hasConsole = hasConsole,
+      isDumbTerminal = isDumbTerminal,
+      isCI = isCI,
+      isEmacs = Util.isEmacs,
+    )
   private lazy val isColorEnabledProp: Option[Boolean] =
     sys.props.get("sbt.color").orElse(sys.props.get("sbt.colour")).flatMap(parseLogOption)
   private[sbt] lazy val isColorEnabled = useColorDefault

internal/util-logging/src/test/scala/sbt/internal/util/TerminalColorSpec.scala

@@ -60,4 +60,54 @@ object TerminalColorSpec extends BasicTestSuite:
     val output = out.toString("UTF-8")
     assert(output.contains(ESC))
     assert(output.contains("red text"))
+
+  // isColorDefault pure function tests
+
+  private def colorDefault(
+      propsColor: Option[Boolean] = None,
+      colorProp: Option[Boolean] = None,
+      logFormatOpt: Option[Boolean] = None,
+      hasConsole: Boolean = false,
+      isDumbTerminal: Boolean = false,
+      isCI: Boolean = false,
+      isEmacs: Boolean = false,
+  ): Boolean =
+    Terminal.isColorDefault(
+      propsColor,
+      colorProp,
+      logFormatOpt,
+      hasConsole,
+      isDumbTerminal,
+      isCI,
+      isEmacs
+    )
+
+  test("isColorDefault should use propsColor when set"):
+    assert(colorDefault(propsColor = Some(true), colorProp = Some(false), hasConsole = false))
+    assert(!colorDefault(propsColor = Some(false), colorProp = Some(true), hasConsole = true))
+
+  test("isColorDefault should fall back to colorProp when propsColor is None"):
+    assert(colorDefault(colorProp = Some(true), hasConsole = false))
+    assert(!colorDefault(colorProp = Some(false), hasConsole = true))
+
+  test("isColorDefault should use logFormatOpt when props and colorProp are None"):
+    // logFormatOpt = Some(false) vetoes color even with console and CI
+    assert(!colorDefault(logFormatOpt = Some(false), hasConsole = true, isCI = true))
+    // logFormatOpt = Some(true) allows color when terminal heuristic is positive
+    assert(colorDefault(logFormatOpt = Some(true), hasConsole = true))
+
+  test("isColorDefault should enable color for interactive console"):
+    assert(colorDefault(hasConsole = true))
+
+  test("isColorDefault should disable color for dumb terminal without CI"):
+    assert(!colorDefault(hasConsole = true, isDumbTerminal = true))
+
+  test("isColorDefault should enable color on CI"):
+    assert(colorDefault(isCI = true))
+
+  test("isColorDefault should enable color in Emacs"):
+    assert(colorDefault(isEmacs = true))
+
+  test("isColorDefault should disable color when no console, no CI, no Emacs"):
+    assert(!colorDefault())
 end TerminalColorSpec