the colour of ferric oxalate depends on the amount of oxalic acid it contains. Without or with very little oxalic acid it is brownish. This is best for kallitypes. For pl/pd, you want oxalic acid inside, because prints clear easier this way, and the more oxalic acid, the greener the colour.
The problem with (over-)heating FO is that some of it may deteriorate, that is change into ferrous oxalate. This is also what may happen if the solution becomes old. The solution for such a solution (couldn't resist;-)): drop some peroxide into it, which will turn any ferrous oxalate back to ferric.
By the way, I usually make my own FO from ferrous by adding oxalic and peroxide (which induces the reaction); this way I get the colour I want without any heating in first-rate quality.