If you aren't offering any payment, I doubt people will be lining up to volunteer for this - most of us are working on our own comics, and if we're going to be working for free, we're much better off working on our own stuff.
You will have a slightly better chance of getting someone to consider your project if you give them more info about it. How many pages is it? When do you want it to be done? How often do you plan on releasing pages? Saying "flexible schedule" is necessary is all well and good, but since you're asking people to spend their spare time for free on this, I don't think you can ask people to bend their schedules to fit yours - this is going to have to happen in whatever time they can spare.
The Venn diagram @cryoclaire posted might be edging into sarcasm, but it's also pretty close to reality; if you want something fast and good, you'll have to pay for it. If you want something cheap and good, you'll have to wait. If you want something fast and cheap, it's probably not going to be very good. And you're asking for something for free, so you can't really make any demands on a stranger to give up their free time to suit your schedule.
Also, final question - what's stopping you from colouring this comic yourself? If you've got a scanner, invest in some watercolours or some coloured pencils and do it yourself. OR, maybe, just do it in black and white. There are also various art softwares available for free, if you want to colour it digitally. Medibang, for example.
