With the tannic acid bath followed by nitric acid, the stock will turn jet black. At that point, you will need to sand it back to the wood to uncover the harder tops and valleys of the curl. The end grain of the curl will retain the darkness and no amount of scrubbing will take it out. At that stage, another application of FN and a blush will yield the brown/red colour we associate with longrifles. Studying books like the Moravian book will show that the red colour we aim for didn't exist in those early examples...it was simply brown. Mike nailed it in his example.