diff --git a/docs/source/release/v6.12.0/Workbench/Bugfixes/38587.rst b/docs/source/release/v6.12.0/Workbench/Bugfixes/38587.rst
new file mode 100644
index 000000000000..71c748c1d4ee
--- /dev/null
+++ b/docs/source/release/v6.12.0/Workbench/Bugfixes/38587.rst
@@ -0,0 +1 @@
+- The colorbar (used in Sliceviewer) has been updated to again allow the registration of custom matplotlib colormaps in scripts.
diff --git a/qt/python/mantidqt/mantidqt/widgets/colorbar/colorbar.py b/qt/python/mantidqt/mantidqt/widgets/colorbar/colorbar.py
index 520e470a1fc3..fa8a667f5f14 100644
--- a/qt/python/mantidqt/mantidqt/widgets/colorbar/colorbar.py
+++ b/qt/python/mantidqt/mantidqt/widgets/colorbar/colorbar.py
@@ -45,15 +45,16 @@ class ColorbarWidget(QWidget):
     scaleNormChanged = Signal()
     # register additional color maps from file
     register_customized_colormaps()
-    # create the list
-    cmap_list = sorted([cmap for cmap in colormaps.keys() if not cmap.endswith("_r")])
 
     def __init__(self, parent=None, default_norm_scale=None):
         """
         :param default_scale: None uses linear, else either a string or tuple(string, other arguments), e.g. tuple('Power', exponent)
         """
 
-        super(ColorbarWidget, self).__init__(parent)
+        super().__init__(parent)
+
+        # create the list. Initialize in the init so that it can be updated if new colormaps are added
+        self.cmap_list = sorted([cmap for cmap in colormaps.keys() if not cmap.endswith("_r")])
 
         self.setWindowTitle("Colorbar")
         self.setMaximumWidth(100)
@@ -181,7 +182,11 @@ def set_mappable(self, mappable):
         self.colorbar = Colorbar(ax=self.ax, mappable=mappable)
         self.cmin_value, self.cmax_value = mappable.get_clim()
         self.update_clim_text()
-        self.cmap_changed(cmap, False)
+        try:
+            self.cmap_changed(cmap, False)
+        except ValueError:
+            # the default mantid colormap is not available, just use matplotlib default
+            pass
 
         mappable_cmap = get_current_cmap(mappable)
 
diff --git a/qt/python/mantidqt/mantidqt/widgets/colorbar/test/test_colorbar.py b/qt/python/mantidqt/mantidqt/widgets/colorbar/test/test_colorbar.py
index a1b23acb3729..5d26e5efc274 100644
--- a/qt/python/mantidqt/mantidqt/widgets/colorbar/test/test_colorbar.py
+++ b/qt/python/mantidqt/mantidqt/widgets/colorbar/test/test_colorbar.py
@@ -13,7 +13,7 @@
 
 from mantidqt.utils.qt.testing import start_qapplication
 from mantidqt.widgets.colorbar.colorbar import ColorbarWidget, NORM_OPTS
-from matplotlib.colors import LogNorm, Normalize, SymLogNorm
+from matplotlib.colors import LogNorm, Normalize, SymLogNorm, ListedColormap
 
 
 @start_qapplication
@@ -239,3 +239,18 @@ def test_invalid_cmin_syntax_is_reset(self):
             self.widget.clim_changed()
 
             self.assertEqual("0.0", self.widget.cmin.text())
+
+    def test_custom_colormaps(self):
+        # test that users can register custom colormaps and have it as an option in the colorbar
+        cmap_name = "custom_cmap"
+        self.assertFalse(cmap_name in plt.colormaps)
+        self.assertTrue(self.widget.cmap.findText(cmap_name) == -1)
+
+        plt.colormaps.register(cmap=ListedColormap([[0, 0, 0], [0, 0, 1]]), name=cmap_name)
+        self.assertTrue(cmap_name in plt.colormaps)
+
+        # now when you create a colorbar widget, the custom colormap should now be available
+        cb = ColorbarWidget()
+        self.assertTrue(cb.cmap.findText(cmap_name) != -1)
+
+        plt.colormaps.unregister(cmap_name)