after days of trying to get a handle on GIMP image editing program, i have finally figured out how the Nintendo DS color palette works in regards to DS Icons. DS icons are 32 x 32, 15 Colors, with an extra, 16th color relegated to a transparency layer. a good example of this is Okamiden. the unmodified icon of that game has white set as the transparency color, so the Icons background is transparent. also, the DS uses different color palette. so many colors just won't display correctly on a DS.
i created a complete Colormap of the 256 Colors used in Nintendo DS icons, that can be used in GIMP.
https://www.mediafire.com/file/8m9zlogtjt221ai/Nintendo-DS-Colormap.gpl/file
the process for creating custom DS icons is a lot more complicated that i had initially thought. you basically have to create a new Color Palette for every icon, which isn't as bad as it sounds since you only need to select 15 colors plus a final transparent color.
just to quickly outline the method i found easiest, starting with a 32 x 32 base icon image, in GIMP, open the color palettes box with Windows, Dockable Dialogs, Palettes.
then Image, Mode, Indexed. ... Use Custom Palette. check remove unused colors from colormap. this will convert the image to only use Nintendo DS compatible colors.
select "New Palette". then use the color picker / eye dropper tool to add 15 colors, adding new entries from the currently selected foreground color. the last color, #16, is a transparency layer. choose some nonexistent color that's not in the image to make sure there will be no invisible colors. Bright Neon Green or some color that wont interfere with the image. click save.
then go to Edit, "Undo Convert Image to Indexed", (you could also just close the window discarding changes and open the image again). then, go back to image, mode, indexed again, this time choosing the new 16-color palette you just created. (uncheck remove unused colors from colormap option)
now that the image is converted to 32 x 32, 16 colors, you can "Export" and save it somewhere, then it's simply a matter of using "DSBuff" to replace the Icon.
i created a complete Colormap of the 256 Colors used in Nintendo DS icons, that can be used in GIMP.
https://www.mediafire.com/file/8m9zlogtjt221ai/Nintendo-DS-Colormap.gpl/file
the process for creating custom DS icons is a lot more complicated that i had initially thought. you basically have to create a new Color Palette for every icon, which isn't as bad as it sounds since you only need to select 15 colors plus a final transparent color.
just to quickly outline the method i found easiest, starting with a 32 x 32 base icon image, in GIMP, open the color palettes box with Windows, Dockable Dialogs, Palettes.
then Image, Mode, Indexed. ... Use Custom Palette. check remove unused colors from colormap. this will convert the image to only use Nintendo DS compatible colors.
select "New Palette". then use the color picker / eye dropper tool to add 15 colors, adding new entries from the currently selected foreground color. the last color, #16, is a transparency layer. choose some nonexistent color that's not in the image to make sure there will be no invisible colors. Bright Neon Green or some color that wont interfere with the image. click save.
then go to Edit, "Undo Convert Image to Indexed", (you could also just close the window discarding changes and open the image again). then, go back to image, mode, indexed again, this time choosing the new 16-color palette you just created. (uncheck remove unused colors from colormap option)
now that the image is converted to 32 x 32, 16 colors, you can "Export" and save it somewhere, then it's simply a matter of using "DSBuff" to replace the Icon.
Or others 
