Yellow is still ok but it is agglomeration. Larger particles.
Their is no reason to heat the water other than to speed up the reaction in the beginning. When you first start with distilled water their is almost no conduction. Heating increases the conduction of pure distilled water. You could speed things up a bit by adding some silver from a previous batch. But I don't bother because I could care less how long it takes to make the silver. I have the auto no heat magnetic stirrer.