Hi.

I have a background picture within its own layer that is locked.

Then I have another layer above where I have a beizer curve. There is a colored line on that picture that I want to apply same color to my beizer curve. So I guess I could use the for that job.

However - I don't know how to do that

It's NOT like ms_paint I figured out.

Select your line, choose the Color Picker tool and Shift + click on the color you like, zoom in if necessary to be precise. Shift makes sure the color is applied to the stroke and not fill of your object.

The difference with MS Paint is that in MS Paint you use the color picker to change the "active" color to whatever you click on. Inkscape doesn't really have an idea of an active color. In Inkscape, you use the color picker to change the fill or stroke color of the currently selected object. So you need to select the object you want to change first, then use the color picker to click on the color you want.

Excuse me if this is a dumb question, I'm just getting started here. What if the color you want is in a different document? Do I just need to insert the image for a second to get the color I want or is there a better way?

If you want to pick fill from a vector object and not a raster image, you can copy the object in one document (Ctrl+C) and select the object in the other file and paste in the style (Ctrl+Shift+V).

Similarly, if your original object was a raster image/boundled in a group/on a locked layer etc., draw a rectangle in that file, use the dropper tool to pick a fill/stroke colour to it, then cut it (Ctrl+X), and paste in the style in the other svg -Ctrl+Shift+V, when target object is selected.