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I found it's tricky to colorize a sub-expression of an expression that changes to a different expression when it is evaluated.

For example, if I replace s with Style[s,Red] in $$ \sum_{k=1}^{n}k^{-s} $$, I get HarmonicNumber[n,s] with s colored red.

The solution I found is to evaluate Style[s,Red] in another line and use copy-paste to replace the s in the original expression.

Is there any better way?

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2 Answers 2

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Use an Inactive Sum

expr = Inactive[Sum][k^-s, {k, 1, n}] /. k -> Style[k, Red]

enter image description here

However, this form cannot be directly activated

expr // Activate

enter image description here

Convert back prior to activating

expr /. Style[k, Red] -> k // Activate

(* HarmonicNumber[n, s] *)
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This produces an input cell that can be evaluated:

CellPrint@ExpressionCell[
  Defer@Sum[
    Style[k, StripOnInput -> True, FontColor -> Red]^-s, {Style[k, 
      StripOnInput -> True, FontColor -> Red], 1, n}],
  "Input"]

Mathematica graphics

[It basically constructs input equivalent to selecting each k in the sum and using the menu Format > TextColor > Red to change the color, which is what I did in the comment I left under the question.]

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