At times the top layer of the batter turns yellow. This usually doesn't happen when the idli batter is kept to ferment on the counter top or even in the Instant Pot. This is more likely to happen, when the batter is kept in the oven with the oven light on (for me, anyway). Though it doesn't necessarily mean that the idli batter has gone bad. More often than not you can just mix everything together after the batter has fermented, and use it as you would normally do. If you want to, you can even scrape off the top layer, and discard it. But that is usually not required. As many a times this is merely because of the kind/quality of urad dal.