I'll take a shot at your question.
Short answer is WHITE leather pack straps and cross belts for the 27th (Inniskilling) Foot.
I too have a 15mm battalion of the 27th Foot in my British forces. I painted the leather cross belts of the soldiers in my 27th battalion white; just like all the rest of my British heavy infantry soldiers. One of my source books, "Uniforms of Waterloo in Colour" by Philip Haythornthwaite, Plate 12, shows the sergeant of the light company, 27 (Inniskilling) Regiment with clearly white leather pack straps and cross belts. Another of my source books, "1815 The Armies of Waterloo" by Ugo Pericoli, Plate 101, shows a sergeant major and a sergeant of the 27th (Inniskilling) grenadier company also with white pack straps and cross belts. In the descriptions of all British heavy infantry, such as the 27th Foot, the differences are detailed but never have I found that the leather pack straps and cross belts were anything but white. Black is the only other color I can find for pack straps and cross belts; such as rifle infantry.
John, I suspect the artist that you are referencing using buff colored leather was trying to show the leather as weathered and not recently chalk whited. Such would not be the case when soldiers were freshly turned out in garrison on parade. I may be wrong here, but I believe a white chalk block was one of the issued items expected to be in the soldier's back pack.
Hope that helps. Maybe others with more and better information will comment.