Update text_color_for_bg helper to support RGB triplet color codes

Closes #12677

app/helpers/labels_helper.rb

@@ -83,7 +83,11 @@ module LabelsHelper
   end
 
   def text_color_for_bg(bg_color)
-    r, g, b = bg_color.slice(1,7).scan(/.{2}/).map(&:hex)
+    if bg_color.length == 4
+      r, g, b = bg_color[1, 4].scan(/./).map { |v| (v * 2).hex }
+    else
+      r, g, b = bg_color[1, 7].scan(/.{2}/).map(&:hex)
+    end
 
     if (r + g + b) > 500
       '#333333'

spec/helpers/labels_helper_spec.rb

@@ -66,5 +66,10 @@ describe LabelsHelper do
     it 'uses dark text on light backgrounds' do
       expect(text_color_for_bg('#EEEEEE')).to eq('#333333')
     end
+
+    it 'supports RGB triplets' do
+      expect(text_color_for_bg('#FFF')).to eq '#333333'
+      expect(text_color_for_bg('#000')).to eq '#FFFFFF'
+    end
   end
 end