Oh, that's interesting.
So if I were to opt for one of the quite nice looking composite doors with their authentic looking woodgrain, the frame itself will just be uPVC and so without the authentic looking woodgrain effect.
And, as inferred above, if the door is one particular brand, the frame will be something else i.e. Rehau, Veka, Spectus etc...
So it's almost certain the frame won't perfectly match the door colour. I do prefer the door colour matching the frame colour, but if they're not essentially the same product then I can see why you recommend contrasting colours Gazman16.
Why don't the manufacturers of expensive composite doors also make matching composite frames? It seems like an obvious thing to do.