When you buy a new Pingi bag, the product contains no moisture and is fully blue. When you recharge in your own microwave, a percentage of the moisture remains (surprisingly it varies between microwave brands/models). This is normal and no problem, but causes the indicator to be a little less vibrant in color. If you notice that recharging in your microwave takes a bit longer, you can wait 5 minutes and do a second cycle of 3 minutes.
It is also normal for the indicator to gradually become less vibrant over time.