The [Gray] button toggles to an a gray slider panel:

This saves making three adjustments to get gray, which is the sensible colour to choose in combination with the luminosity blend effect. The [RGB] button switches back to RGB entry mode. It doesn't matter which panel is active when you save the settings. RGB values are automatically kept in sync with the gray value.

Position

There are six positions for the signature:

1. Top left corner running horizontally
2. Top left corner running vertically down
3. Bottom left running horizontally
4. Bottom right running horizontally
5. Bottom right running vertically up
6. Top right running horizontally

Just click on the radio button for the position you want. Here's a blank image with all six positions shown and another set of examples showing the effect of different offsets: Screenshot of positions