Looking at the colors and curl.
Do not panic!!!
The ferric nitrate (nitrate of iron) stain will darken with age. The "curl" has picked up a good deal more stain than the grain nearly parallel to the surface.
The overall color will become a bit darker and the curl should turn nearly jet-black within 10 years of aging.
The wood has some tannic acid in it that is "natural" to the wood. Then you put a finish on the wood. The finish "breathes" allowing moisture to enter or leave the stock. Finishes are not 100% vapor barriers. Their function is to simply moderate or control the rate and amount of moisture change in the wood.
The tannic acid, in the wood, is water soluble. As water migrates out of the wood it runs into the finish on the surface. Leaving the tannic acid behind as it enters the finish and then evaporates off the surface. That tannic acid reacts with the iron oxide created by the stain. The curl will begin to darken and then turn black because of the high iron content in that portion of the wood. In addition. Moisture (water) travels with the grain in the wood. The so-called wood fibers are actually tubes. What the tree had used to transport water from the roots to the leaves. S9o you get a higher concentration of iron and tannic acid in the curl which is really end grain.
The color change takes time and how long has a lot to do with where you live and what you see in day to day changes in relative humidity.
This reaction between iron and tannic acid had long been seen in writing inks before the introduction of steel pen points. The reaction between iron and tannic acid had long been used to dye fabrics.