Not a bug
BYOND Version:471
Operating System:Windows XP Pro
Web Browser:Firefox 3.6.6
Applies to:Dream Maker
Status: Not a bug

This is not a bug. It may be an incorrect use of syntax or a limitation in the software. For further discussion on the matter, please consult the BYOND forums.
The bug:
Colors Used in video:
(White / rgb(127,127,127) / rgb(128,128,128) / Black)

Video: (Size: 7.6MB)

At the time of the video I thought it was only grayscale colors, but it applies to every color.

1.) Create a blotch of one color on an icon.
2.) Create a blotch of another color.
3.) And Another (If you want)

4.) click on one of the colors you have added inside of the "Colors in use" area and press delete.

Actual Results:
Every color used in that state get replaced with what you have right-clicked in the used area.

Expected results:
The colors which you have selected, get replaced with that which you have right clicked (default: Mask)

Credits: Inushi & Myself
Pressing Delete in an icon_state frame deletes the content of that frame. To delete colors you have to go to Colors > Edit... from the file menu
Maybe, but it seems like it would be much more useful to do this inside of the icon state, using the colors in use as a selector left click -> right click on pressing delete