Hi,
I dont use any acrylic or humbrol color undercoat.
I undercoat my figures entierely with humbrol. The colors are grey, ocre, brown.... any of which I have (it does not matter). The only important thing is to undercoat with old humbrol. The ones without the rainbow paint on the tin.
I paint with oil directly over my undercoat.
Sometimes I pass a Burnt Sienna thinned with white spirit.
JP