Breed them true, or cross the rooster with a black hen.
If I am not mistakes, In Polish you breed a Khaki to a Khaki you get Khaki, but the color gets lighter after a while . So you breed them to a chocolate, and get half & half. since there is no chocolate silkies that I am aware of I would use a Black silkie hens to your rooster. If you breed them togeather you might get more khakis or maybe even chocolate silkies.
If not when breeding to a black hen, take all your roosters offspring/ daughter and breed them back with him.
If you use a black silkie hen you might get chocolates or more Khaki silkies.
But again for now I would breed them true if at all possible . & if you have the time expirement with crossing in black hens.
Bretty much Like if you were Breeding Blues.
Khaki Roo x black hen 1st generation
Breed father x daughters 2nd generation, to strengthen the khaki gene.