I can't make it work, either.

But there's a trick for a workaround:

With the dropper tool (when it has not been started from the dialog, but via shortcut or its button in the tool box), hover over the color you want, then press Ctrl+C. The color is now in your clipboard, and you can paste it into the field.

I seem to recall that this has been reported already, but that might have been in the old bug tracker.

So I'd suggest that:

1. You try if the bug persists with the beta version (beta2 is about to be released)

2. If it does, you make a bug report at https://inkscape.org/report

I tested it and its a bug (0.94 and also 1.0). I think it's probably because after you open this type of filter dialogs you cannot access canvas or canvas tools. Report it