diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -1716,7 +1716,7 @@ static void ViewportDrawStrings(ZoomLeve
 				/* Don't draw the rectangle.
 				 * Real colours need the TC_IS_PALETTE_COLOUR flag.
 				 * Otherwise colours from _string_colourmap are assumed. */
-				colour = (TextColour)_colour_gradient[ss.colour][6] | TC_IS_PALETTE_COLOUR;
+				colour = (TextColour)GetColourGradient(ss.colour, 6) | TC_IS_PALETTE_COLOUR;
 			} else {
 				/* Draw the rectangle if 'transparent station signs' is off,
 				 * or if we are drawing a general text sign (STR_WHITE_SIGN). */