Actually for colors, this formula is better:

if(isnull([qID_15876_Last step in cart]), rgb(100,100,100),

pick(Match([qID_15876_Last step in cart],'Left in cart','Checkout','Personal info','Payment info','Reviewed, no confirm'),

rgb(255,181,128), rgb(255,151,77), rgb(255,113,13), rgb(217,97,11),rgb(179,85,18)))