I'm no expert on different fungal effects.
The thing we call Spalting with the dramatic dark lines is just one of type of fungus. I've had timber that had spent several years in a firewood pile with the finest filigree lines of spalting in it. I personally think of these two as Wet and Dry spalting (bound to be totally wrong).
In America you can certainly buy fungal spores which will create different colours as it infects the wood. I found details about this from an American University study, available on-line, very interesting.but of course we cannot import them.
Here in the UK you can by Mushroom plugs (
https://www.gourmetmushrooms.co.uk/)to knock into fresh cut timber, to grow edible mushrooms from. I've never tried it but the mycelium must affect the wood for colour or spalt lines of some sort.
There are many bracket Fungi that affect growing trees in many ways, Beefsteak Fungus turns Oak into Brown Oak. You mention Birch, they can be affected by the birch Polypore or Horse-hoof (or tinder) fungus, it might be that your tree was infected prior to you cutting it and hence it turned at an accelerated rate.
