Use a loop with CPChar, CPAttr, CPMoveNext

var col : Integer;
begin
  col := WPRichText1.CurrAttr.ColorToNr(clRed, true);
  WPRichText1.CPPosition := 0;
  repeat
    if WPRichText1.CPChar in ['A'..'Z'] then
    begin
       WPRichText1.CPAttr.BeginUpdate;
       WPRichText1.CPAttr.Color := col;
       WPRichText1.CPAttr.IncludeStyle(afsBold);
       WPRichText1.CPAttr.EndUpdate;
    end;
  until not WPRichText1.CPMoveNext;
end;