Kodak Endura paper uses a TiO2 (Titanox) impregnated layer for whiteness. In that layer are many chemical preservatives and tints to make the Titanox as white as possible. The paper contains a UV absorber to reduce dye fade. The paper also contains an optical brightener which is present in the color developer made by Kodak (IDK about others).
The preservatives tend to make the paper yellowish and the brighteners tend to remove that color and whiten the print.
However, fog can cause stain in prints. Any print processed in a substandard manner or with a substandard process will have increased stain which is increased by the age of the stock you use.
In the final analysis, all color papers will have a mild Dmin that has a density of about 0.15. The stain should be neutral if things work right! A stain of 0.2 or higher indicates a problem.