You don't need to use outlines. I agree with others, you should draw in a style that speaks to you and makes you happy. There's all different types of styles to work in, doing something different will make your work stand out, and people able to recognize it as yours!
I think you have a lot of skill, your artwork is beautiful and detailed. Really great job with all your characters, proportions, backgrounds-! There's a lot of amazing stuff here, even just from the examples you gave! If I had advice I would say to look up tutorials on color theory and characters in environments.
What it looks to me is that you are focusing on local colors. You seem to be painting each object exactly as the color that it is, without effects of environment or lighting. That's a great place to start! The unfortunate thing is that it makes everything in the image the same level of importance. When coloring, lighting and contrast are your tools to lead the eye and show the viewer what to look and at focus on: otherwise the eye will be confused and possibly give up, making it difficult to appreciate all the hard work you've put into your panels!
With a quick search I found this tutorial, which might give you a starting point:
and here's another one that might help too.
Color theory & color harmony are great things to search- color is such a powerful tool to set a mood and give a lot of information about a scene quickly!
I agree that looking at other artists who do a lineless style, and how they make it work for them, will help a lot. I think that Samurai Jack is a great resource, and always uses contrast and limited color pallets to great effect. Even just compare the top images you posted in the thread with your comic panels- your lineless stuff really pops because you use simple backgrounds and dynamic contrast to show the characters. Maybe incorporate more of that into your comic panels themselves?
Hopefully some of these thoughts help. Regardless, keep up the good work! Keep honing that style that is uniquely yours and pursuing what speaks to you!