Vista - TIFF file highlight area C# example?

I need some sample .NET sample code that allows me to load a TIFF file
via System.Drawing.Bitmap bitmap or TiffBitmapDecoder decoder = new
TiffBitmapDecoder. I'm lost trying to tie all the pieces together. I
know I need to convert the tiff into a bitmap so I can use setpixel
correct? The method will pass in the x,y coord. of the area of the
TIFF file and a color to change it to. My TIFFs are black and white
so I was told I need to convert it to grayscale in order to change the
pixel color to light green for example. Basically I want to change
all the white pixels to a light green color in one specific area of
the image.

Can someone show me a sample code snippet that would accomplish this
using .NET C#, my project is a wpf application.

thanks!