I don't see a problem with outlining the characters only sometimes, if there's a stylistic purpose. Sometimes I use white outlines to give a sense of being emotionally isolated or seperate:
And sometimes it's just a technical/clarity thing, like a black-haired character on a black background ;d
But it's largely situational. Since your comic is in colour, you can use it to your advantage to help the characters pop from the background, usually by having the background colours a little less saturated or dark.
In general though, I'd say just keep practicing the foundations of anatomy and perspective. It'll add a whole lot more clarity to your artwork and you shouldn't have to rely on outlines so much. :>