procedure RESetTextColor(p_theRE: TRichEdit; p_nColor: TColor;
 p_nFrom, p_nTo: Integer);
var
 n: Integer;
begin
 n := p_theRE.SelStart;
 p_theRE.SelStart := p_nFrom;
 p_theRE.SelLength := abs(p_nFrom - p_nTo) + 1;
 p_theRE.SelAttributes.Color := p_nColor;
 p_theRE.SelStart := n;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
 RichEdit1.Text := 'Пример для Kbyte.Ru';
 RESetTextColor(RichEdit1, clBlue, 1, 4);
end;