They look great! I think you're just experiencing standard artist's dismay. Trust me, I'm an artist: these lines were born to be colored. Course, I love color art. I think it might look nice with a more pastel, limited color scheme though.
Backed your Kickstarter 'cause I NEEEEEEEEEEEEEEEEED that Dreamweaver High Priestess card. Seriously. I need it. You have no idea. I have an obsessive compulsion to buy dream themed things as it is, but I have drawn one tarot card... and it was my Dream aligned character as the High Priestess. I would buy a print but I don't have a hundred dollars to back you with to get the print at the moment.
Must have. (twitches) If you don't get funded, seriously, PM me because I'll still buy a print of it off of you.
Anyway colors! Don't worry, your lines definitely want color. The last card there by far works the best. I think the issue you're seeing is that the colors aren't really pulled together. Especially on the Pluto card, the saturation is very severe and I feel like you're angling for a soft, dreamy Art Noveau deck instead. It might look to you like you just dropped colors into the lines, and it stands out too well against the background. I have a cheap and dirty trick for this, using textured overlays to bring everything together as one unified image under the same kind of tone. Another "cheap" trick is to color the lineart a little bit so it meshes better. Softening harsh digital colors and simulating more traditional art look colors is kind of my specialty, so if you want to talk color, just shoot me a private message so we don't clutter up your thread.