How to solve color difference?

In sourcing glass with silkscreen print, customers often face problem about color difference from each batch or even single batch especially white color, why for that? 

Below are the reasons to cause it(take white color for example because of most obvious issue):

1.Source, for each batch most of customers aren't only focus on pure white but others(like signal white) or according to color samples.

   In this case, the ink supplier should match color per formulate but there will be deviation that cause color a little difference.

2.Glass, the glass usually separate into tin side and non tin side, non tin side has more stable color during print.

3.Printing, different screen meshes & printing force define layer thickness.

4.Drying, the drying temperature and time may cause like white yellowish or not. 

   For ceramic ink, the tempering also cause same issue.

5.Condition, as time pass and storage condition difference yellowish will also happen.

So the problem really exist, how we can judge color OK or not?

1.Upper and lower limits:

Request supplier to offer upper and lower limit color samples(acceptable difference), every batch should control within it.

2.Colorimeter:

Sign golden sample as standard, the Delta E(ΔE) should be controlled in certain value(like ΔE ≤ 1.0).

3.The ink supplier(and formulate), screen mesh, layer, time, drying condition and storage condition should be filed as specification.