Are the formless jhanas without any consciouness of form at all?

I don't understand the formless jhanas. Does this meditator completely lose consciousness of form, or are they meditating on something formless and, apart from that, as before? I've read the abhidharmakosa bhashyam, of Vasubhadhu, and he says that a mediator of this sort will be later reborn into a realm without form, so that suggests that there are sentient beings without any of the sense consciousnesses. But I just find the idea impossible, to be honest. Interesting, one of those books says that some Buddhists claim that there is a residue of visual consciousnesses there. Can anyone describe to me what the formless jhana is like? I'm especially intrigued as to whether there is any sense of ***shade*** to it: if it seems darker or lighter than any given colour I've experienced.