Cannot change color in RGB file converted from TIFF Bitmap

I have a series of TIFF files that are bitmap files black on white. I open them convert them from bitmap to Grayscale and then to RGB using image mode. I then want to change the black color to a blue and the background white to a straw color. I have tried in many ways to use the adjust color/replace color option. The correct color shows up in the box in the lower right but when I click ok and ok either nothing happens to the actual background or it may turn a shade of gray.

Any Ideas of what I may be missing?

Dave Herrig
newuser