When the pencil sketch is finished, my next step is usually to block in big areas of foundational color with wet-in-wet washes. Sometimes I do this with gouache, but gouache has a tendency to lift off the paper if it's re-wet. This picture is going to involve lots of layering, so the first washes are going to be of ink. Once ink is down, it stays down.
This image shows a few of those first layers. The palette is very limited--scarlet red and indigo blue acrylic inks and india ink, all diluted to varying degrees with distilled water.