The shipmen - The sailors, leaving the prisoners.
Under colour - Under pretence. They pretended that it was necessary to get into the boat, and carry the anchors ahead of the ship so as to make it secure, but with a real intention to make for the shore.
Out of the foreship - From the prow, so as to make the fore-part of the ship secure. The reason why they did this was probably that they expected the ship would go to pieces; and, since all on board could not be saved in one small boat, they resolved to escape to a place of safety as soon as possible.