i must be a lot more ignorant than i imagined. i had the fg/bg colour dialog open and suddenly noticed something i didn't recognise:

i've used gimp off and on for many years (first time back in the late nineties, it looked a bit different back then), and i still don't know and since it's very likely something everyone else knows i'm feeling really stupid .

Just wondering, did you use a foreground to transparent gradient recently?

I get that when i use the eye dropper to pick a color

I think that is what it is too alc. I use that eye dropper all the time and I have quite a few of those half colored/transparent thangs.

In addition to the RGB values, a color can have an opacity associated with it. While neither the Color Chooser or FG/BG Color dialogs provide a means of setting the opacity directly, you can pick such colors from an image by holding down the SHIFT key when using the Eyedropper Tool. In the pop-up info box that appears, you can then drag-n-drop such "color + opacity"to or from the color swatches which support them.

In addition, you can drag-n-drop such colors from the "Edit Attributes" dialog for channels. You can open an "Edit Attributes" dialog on a channel by either right-clicking on the Quick Mask button (in the lower left of the Image window) or by right-clicking on a channel in the Channels dialog. This dialog will also contain a "color + opacity" swatch which you can use as a drag-n-drop target or source, and the opacity setting will be set in addition to the RGB color.

saulgoode: thank you! i had no idea about any of this, seems like i have to explore a bit here

A different implementation can be found in the Gradient Editor, which has a slider for the opacity.

Nice catch.