grain filling problem

Dear grain fillers,
I'm shellacing an oak table with very open grain and im having a problem with grain filling. The table has already recieved a brown stain (alcohol based) and a thin wash of shellac on top of that in order to keep the grain articulated from the surrounding wood when it recieves the tinted filler. The desired result in this case is that the grain should be darker than the surrounding wood. I'm using a product called Rustin's grain filler which is a 'natural' color, oil based filler. I added brown pigment to the filler and thinned it with white spirit to the desired consistancy. When I applied the filler it looked fine but then it dulled and some of the grain has light colored spots. I'm aware that filler dulls as it dries but I want to resore the grain to the color it was when i first applied the filler, that is, darker than the surrounding wood. I noticed that the filler returns to 'wet' color when i passs a rag dampened with white spirit over it. But of course it reverts to the whitish color when it dries. I thought of coating the surface with a light rub of boiled linseed oil before shellacing. Any ideas greatly appreciated.