Please note in natural vegetable tanned leather the color changes itself by ageing and the sides of color # 623 is natural so it cannot be controlled, we can only match the top of the leather i.e. surface of the leather by paint.
However if you take fully painted leather like 3MM bolo dark blue (PAN 19-3925 PTPX) SP-0274 then sides will be also of same color then you will not have this problem.