If you want to add a third gray level or another color to the two level gray, you can write out the existing grayscale colormap by clicking 'w' with the cursor over the grayscale bar. The file for this simple colormap is just two rows of RGB values with three columns for the Red, Green and Blue fractions, respectively. Edit the file with a text editor and add a row. Here's an example adding a pink color to the list:

% more gray_red.1D.cmap
0.70000 0.70000 0.70000
0.40000 0.40000 0.40000
1 0.5 0.5

The last line is the new pink color. Save the file with the extension ".cmap". The easiest way to use it is by selecting the "New" button in the suma object controller menu next to the "Cmp" selection button. Select the colormap you just created. In the object controller, you can flip the colorbar, threshold, etc. using the new colormap.