Hi Martin
You should not be using [...] in the first parameter of the lookup function. try this:
<<Let(Colour Code = _
<<Lookup(_
<<Layer Code.Colour>>,_
<<Get(Layer Code)>>_
)>>_
)>>_
<<Lookup(<<COLORS.Description>>,<<Get(Colour Code)>>)>>
Also, I see you use the spelling color in the old code and colour in the new code. perhaps you should check this.