A few observations...
Matboard manufacturers buy colored top papers, core papers, and backing papers from paper mills in large rolls, and laminate them together to make matboards. Our matboard makers do not impart the color to the papers.
About 200 North American paper mills have closed in recent years, so all of our matboard manufacturers (as well as other users of paper) have had to establish business relationships with new suppliers recently. And, because paper making is a complex process, the buyer needs to be directly involved. It's not only a matter of negotiating a price and placing an order, it's a relationship that requires constant monitoring.
At the paper mill (not the matboard maker) a color is mixed, typically from three or more pigments/dyes. The batched mixture is added to the paper as it is produced in a continuous process. For some mysterious reason, the color tends to be lighter near the end of a production run, even though the pre-mixed color all comes out of the same vat. They call it "color drift". To counteract this phenomenon, a production employee has to monitor the color and make adjustments on-the-fly during production. So, aside from specifying the colors they want to buy, matboard manufacturers have to continuously monitor the colors being produced for their papers by the mills. If the buyer becomes lax, the supplier becomes careless.
Matboard colors usually consist of three or more pigments or dyes. Any variances among them compound the problem of color variations in the final product.
For museum-grade/conservation matboards made of alpha cellulose, the colors are typically produced from pigments that are chemically stable and resistant to light damage. However, the colors of white-core and regular wood-pulp matboards typically are produced from cheaper pigments or dyes that are less stable over time. These cheaper mats might show color deterioration soon, not only from light exposure, but also from chemical reactions with the paper over time.
For color variation issues, the best solution is to buy the best matboard from a manufacturer that buys the best papers from the best paper mils, and monitors their production process most carefully. That's one of the reasons I buy only alpha cellulose matboards made from purified pulp or cotton, and prefer those made by a smaller manufacturer, where the principle owner is closely involved in the process.
