If you create a tabset with the options and specify colors using the english description like "Dark Blue"

this is due to the use of 'eval' and not properly creating the command using [list] to preserve quoting.

Here is the patch you can apply to fix the problem
-------------------------------------------------------------------
--- tabset.itk.orig	2018-06-05 16:23:54.228090301 -0700
+++ tabset.itk	2018-06-05 16:24:40.547745872 -0700
@@ -893,11 +893,11 @@
         -left             $x \
         -top              $y \
         -font             [list $itk_option(-font)] \
-        -background       $itk_option(-background) \
-        -foreground       $itk_option(-foreground) \
-        -selectforeground $itk_option(-selectforeground) \
-        -disabledforeground $itk_option(-disabledforeground) \
-        -selectbackground $itk_option(-selectbackground) \
+        -background       [list $itk_option(-background)] \
+        -foreground       [list $itk_option(-foreground)] \
+        -selectforeground [list $itk_option(-selectforeground)] \
+        -disabledforeground [list $itk_option(-disabledforeground)] \
+        -selectbackground [list $itk_option(-selectbackground)] \
         -angle            $itk_option(-angle) \
         -padx             $itk_option(-padx) \
         -pady             $itk_option(-pady) \