Hi Sandy...I haven't found "denim" but here are some blue ratios:
From Whimsical Bakehouse (using liquid gel colors per 1/4 C buttercream):
ice blue = 1 drop blue
sky blue = 3 drops sky blue
royal blue = 5 drops blue
midnight blue = 5 drops blue + 4 drops purple
periwinkle = 1 drop blue + 1 drop purple
From Wilton "Decorating Cakes:
Navy Blue = Royal blue + small amount of black
(no specific amounts were given: "keep adding a drop with a toothpick and testing color.")
I think most any blue will work with the stitch work seams in white.
