How to count the amount of white Pixels time efficiently in C#

Question

I am trying to compare two bitmaps to one another. One is premade, and the other one consists of a small image of the main screen being taken and filtered for everything besides full white. I now need a way to compare the amount of white pixels in the live Bitmap to the amount in the premade Bitmap (101 white pixels). A way I know of would be using the Bitmap.Get/SetPixel commands, but they are really slow and as this is used for a kind of time critical application, unfitting. Especially since I could cut down the filtering process by a factor of 70 by following this guide.

https://www.codeproject.com/articles/617613/fast-pixel-operations-in-net-with-and-without-unsa

I also can't just compare the 2 Bitmaps, as the live one will usually not have the pixels in the same position, but will share the same amount of white pixels.

So yeah. It'd be great if one of you had a time effective solution to this problem.

Edit
Huge oversight on my part. When looking at the filtering method, it becomes apparent that one can just use a counter+=1 every time a pixel is not filtered out. So I just changed this line of code in the filter function

row[rIndex] = row[bIndex] = row[gIndex] = distance > toleranceSquared ? unmatchingValue : matchingValue;

to this

if(distance > toleranceSquared)
{
row[rIndex] = row[bIndex] = row[gIndex] = unmatchingValue;
}
else
{
    row[rIndex] = row[bIndex] = row[gIndex] = matchingValue;
    WhitePixelCount += 1;
}