You aren't going to get faster at colouring by posting pages in black-and-white. But I'm seconding @gateofselidor -- Which is better for the comic that you want to make? The aesthetics of colour, or the pacing of more frequent updates? That's not a decision anyone can make for you, because one isn't inherently better -- it's about which matters more to you.
If it's just a matter of worrying about getting an update right on time, you might want to look into a buffer, so that getting a page done a little late won't hurt your update. Like, if you've posted page 2, and you have page 3 and 4 finished already, then it doesn't matter if update day rolls around and you need a couple more hours to finish page 5, because page 3 is ready to be posted. If you work this way, you have a little bit of wiggle room, where if you're able to finish a page roughly once a week, it's okay that it's not done right on the dot of update night.
So a more useful question might be, how long does this actually take you, and do you have that much time to colour pages? Like, if it consistently takes you 10 days to colour a page and you want to update every 7 days, then that answers your question. But if it's just a matter of "I can colour a page in 7 days, but when a page gives me trouble I run a little over," then getting a buffer might clear up your worries about running a little late sometimes!