diff --git a/bin/__build_sgr_code b/bin/__build_sgr_code
index a963409..560c27c 100755
--- a/bin/__build_sgr_code
+++ b/bin/__build_sgr_code
@@ -119,10 +119,14 @@ function render_color()
     command_color=30
     if (( ${use_3_bit} ))
     then
-        basecolor=$(( ${command_color} ))
+        # Underline colours need 8-bit or more
+        if (( ${background} > 1 )) ; then return; fi
+        basecolor=$(( ${command_color} + ${background}*10))
         next="$(( ${basecolor} + ${legacy_3_bit} ))"
     elif (( ${use_4_bit} ))
     then
+        # Underline colours need 8-bit or more
+        if (( ${background} > 1 )) ; then return; fi
         command_color=$(( ${command_color} + ${intensity_1_bit} * 60 ))
         basecolor=$(( ${command_color} + ${background}*10 ))
         next="$(( ${basecolor} + ${legacy_3_bit} ))"
@@ -381,8 +385,14 @@ do
         bg:[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]) make_color 1 rgb_color ${1:3} ;;
         bg:[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]) make_color 1 rgb_color ${1:3} ;;
 
+        ul:ext:*) make_color 2 ext_color ${1:7} ;;
+        # 12-bit color also supported
+        ul:[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]) make_color 2 rgb_color ${1:3} ;;
+        ul:[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]) make_color 2 rgb_color ${1:3} ;;
+
         fg:*) make_color 0 named_color ${1:3} ;;
         bg:*) make_color 1 named_color ${1:3} ;;
+        ul:*) make_color 2 named_color ${1:3} ;;
         *) make_color 0 named_color $1 ;;
     esac
     if [[ ! -z ${output} ]]