You could try formatting with MakeBoxes (or maybe with Format). So here f[x,u] is the expression you type and the expression in the FullForm expression, but an InterpretationBox is used to specify the Output form of the expression. The InterpretationBox is applied to a List where the first expression gives the box formatting and the second expression gives the internal expression. (This is done because InterpretationBox has the Attribute HoldAllComplete and we may want to compute things to put in the first expression.) So, for example:
MakeBoxes[f[x_, u_], form : StandardForm | TraditionalForm] :=
InterpretationBox[#1, #2] & @@ {StyleBox[
ToBoxes[f[x] + B[x] Style[u, Red], form], 20, Bold], f[x, u]}
f[x, u]
% // FullForm
You can turn off the formatting with:
MakeBoxes[f[x_, u_], form : StandardForm | TraditionalForm] =.
f[x, u]
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