diff --git a/extra_tests/custom_text_test_runner.py b/extra_tests/custom_text_test_runner.py
index f6f71d7e1..cb0006a74 100644
--- a/extra_tests/custom_text_test_runner.py
+++ b/extra_tests/custom_text_test_runner.py
@@ -36,6 +36,8 @@ import unittest
 from functools import reduce
 from unittest.runner import registerResult, result
 
+from _colorize import decolor
+
 
 def _subtest_id(parent, subtest):
     try:
@@ -247,6 +249,10 @@ class CustomTextTestResult(result.TestResult):
         self.separator_failure = "!" * CustomTextTestResult._num_formatting_chars
         self.separator_pre_result = "." * CustomTextTestResult._num_formatting_chars
 
+    def _exc_info_to_string(self, err, test):
+        # JSON and terminal details share these strings; keep both plain text.
+        return decolor(super()._exc_info_to_string(err, test))
+
     def getDescription(self, test):
         doc_first_line = test.shortDescription()
         if self.descriptions and doc_first_line:
diff --git a/extra_tests/test_custom_text_test_runner.py b/extra_tests/test_custom_text_test_runner.py
index 9aa76cb78..3408057a9 100644
--- a/extra_tests/test_custom_text_test_runner.py
+++ b/extra_tests/test_custom_text_test_runner.py
@@ -2,6 +2,7 @@
 
 import io
 import json
+import os
 import pathlib
 import sys
 import tempfile
@@ -14,6 +15,71 @@ from custom_text_test_runner import CustomTextTestRunner
 
 
 class RunnerResultTests(unittest.TestCase):
+    def test_json_tracebacks_are_plain_under_forced_colors(self):
+        class Cases(unittest.TestCase):
+            def test_failure(self):
+                self.fail("ordinary failure")
+
+            def test_error(self):
+                raise ValueError("ordinary error")
+
+            def test_subtests(self):
+                with self.subTest(kind="failure"):
+                    self.fail("subtest failure")
+                with self.subTest(kind="error"):
+                    raise ValueError("subtest error")
+
+        for color_env in ({}, {"FORCE_COLOR": "1"}, {"PYTHON_COLORS": "1"}):
+            with self.subTest(color_env=color_env), mock.patch.dict(os.environ):
+                for name in ("FORCE_COLOR", "PYTHON_COLORS", "NO_COLOR"):
+                    os.environ.pop(name, None)
+                os.environ.update(color_env)
+                with tempfile.TemporaryDirectory() as temp:
+                    path = pathlib.Path(temp) / "results.json"
+                    log = io.StringIO()
+                    result = CustomTextTestRunner(
+                        stream=log, verbosity=2, results_file_path=str(path)
+                    ).run(unittest.defaultTestLoader.loadTestsFromTestCase(Cases))
+                    data = json.loads(path.read_text())
+                cases = {
+                    case["method"]: case
+                    for suite in data["suites"].values()
+                    for case in suite["cases"].values()
+                }
+                subtests = cases["test_subtests"]["subtests"]
+                tracebacks = [
+                    (
+                        cases["test_failure"]["failures"],
+                        "AssertionError: ordinary failure",
+                    ),
+                    (cases["test_error"]["errors"], "ValueError: ordinary error"),
+                    (
+                        cases["test_subtests"]["failures"],
+                        "AssertionError: subtest failure",
+                    ),
+                    (cases["test_subtests"]["errors"], "ValueError: subtest error"),
+                    (subtests[0]["traceback"], "AssertionError: subtest failure"),
+                    (subtests[1]["traceback"], "ValueError: subtest error"),
+                ]
+                for text, message in tracebacks:
+                    self.assertNotIn("\x1b[", text)
+                    self.assertIn(message, text)
+                    self.assertIn(message, log.getvalue())
+                self.assertEqual(result.testsRun, 3)
+                self.assertEqual(len(result.failures), 2)
+                self.assertEqual(len(result.errors), 2)
+                self.assertFalse(result.wasSuccessful())
+                self.assertEqual(data["num_failed"], 3)
+                self.assertEqual(data["num_passed"], 0)
+                self.assertEqual(data["num_skipped"], 0)
+                self.assertEqual(data["num_expected_failures"], 0)
+                self.assertEqual(cases["test_failure"]["result"], "failed")
+                self.assertEqual(cases["test_error"]["result"], "error")
+                self.assertEqual(cases["test_subtests"]["result"], "error")
+                self.assertEqual(
+                    [sub["result"] for sub in subtests], ["failed", "error"]
+                )
+
     def test_records_parent_and_subtest_outcomes(self):
         class Cases(unittest.TestCase):
             def test_pass(self):