Its not that your hair is ash coloured so much, its that you have a fine smattering of grey light hairs that makes it look more ash.
The stock has none.
So its not just a colour difference, its more of a composition difference.
To get uniformity, you would be better to dye the whole lot in my opinion, but you could start with just your natural hair and see.
Remember that the stock hair will absorb colour a lot faster than your natural hair, so you can't leave it on as long.

I hear people recommending Nice 'n' Easy or Just For Men.
If you go with the JFM, but the AUTO Stop version as it stops colouring when it gets to the right colour.

If its a lace base stock, you can thin the density using Nair Hair remover creme.
There's a few threads on it here and there is a youtube video too.
The mistake most people make is that they think the first application hasn't worked as much as they want, so they do it again.
What happens is that you'll lose some hair immediately with the first application and then some more after a few days/washes.
So don't do a second application until you know the full results of the first one.

You WILL get there in the end.
But you've sort of made life more difficult for yourself by ordering a stock.
They're really only a suitable if they match your specifications.
Some one just starting out doesn't really need or want these added complications on top of everything else!
But I completely understand that you probably didn't want to commit to a custom order until you at least tried a stock.