Dafuq?

neat

You're mistaking water with something else.

because you can paint the inside of the egg for easter with this salt?

You just dodged an assassination attempt. Well done.

I always knew As is harder to detect, although slower. This is a prime example! :P

Could there be starch in the water? Salt is often iodized, iodine reacts with starch to make a blue colour. Maybe you had some potatoes or something starchy in the vessel before?

Edit: Also iodine reacts with glycogen (often refered to as animal starch) to make a red colour. If I had to guess I'd say your vesssel wasn't sterilized, as you only need a trace amounts of the starches to give the colour reaction with iodine.

How to isolate:

1) Try another glass? If not the same, something contaminated the glass.

2) If issue repeats in new glass, try filtered/bottled/other source water. If you can't recreate the issue, it's the water.

3) If the salt colors the water in 2 different glasses with 2 different water sources, then you have contaminated salt. Likely a manufacturing mistake as salt is naturally white and wouldn't need to contain any food coloring or dyes normally.

You said you used water from the tap. Depending on where you live, that could be a factor in it as well. As for the black grains in the salt, that is...strange.

A few questions....1) Was the salt container opened before you went to use it? 2) Where did the salt come from? (e.g. Morton, local store brand etc.) 3) Have you used Google for answers? I did and there is a very real possibility that it is arsenic. Nevertheless, don't use it on your food!

Is it seasalt?

Wow that's pretty messed up. Hope you got some new salt now! As for why that happened... no idea! I'm useless!

This

Over time, iodized salt slowly loses the iodide in it to oxidation (because it is exposed to air). This produces iodine and a carbonate, which apparently are supposed to evaporate. If you've had this salt for a long time, it's possible that the black flecks are leftover from the oxidation process... maybe the iodine and carbonate didnt fully evaporate... I'm no expert though.

Let us know when you get an explanation from the manufacturer :D