LOW OG

[Beermonger]

You'll see purple or black, not blue, when using iodine to test an unconverted mash. The reason is that the wort is dark coloured, unlike a potato...

[Will_D]

The iodine/starch test is very sensitive. If the wort is too dark dilute it! It will still work!

[Drum]

For a dark wort (stout) I put a sample on a white plate and spread it around then add the iodine.  The white background makes it easy to see any colour change