I've done some of this sort of thing myself - also not a pro, but I did notice a few things that I think could help you out. I'll be referencing picture 9 in this set for some of the stuff I'm suggesting here.
I'm not sure what program you're using for this, but if you have a copy of Clip Studio Paint (it goes on sale for like $25 pretty often), I'd recommend running it through their "remove tones" tool, using the "convert to grayscale" feature and seeing if that helps remove the tone shading on some of it in exchange for a smoother blend, and the "pock marks" all over the guy's skin. If it doesn't get rid of those, it may be worth trying to erase them yourself if they stand out like that.
Your edgework could use some touching up, I see a lot of gaps here and there, but overall not bad.
When doing effects like the blush on the girl's face or her nipples, I'm not sure if you use multiple layers or not, but I would suggest it, and then blend / blur the areas around them some. If you're using a mouse it can be a bit tricky, but if you have a tablet then I'd suggest using airbrushes for the effects, and then going over them with a softer eraser to make the blend look better between blush / nipple and base skin tone.
Also on the subject of the nipples, if you see where the tone shading is "missing" a bit, you should use a lighter color for that - again, similar to the edges of the nipples and blush, put it on another layer higher up and just lightly blend it. Tweak the colors and the amounts until you get something that looks nice and doesn't stand out much.
Some of your colors look a bit like they're in conflict with each other. Something I've started doing recently to help with that is making a layer, setting the type to "color," and just making it solid white, and placing it on top of the picture. It basically makes it monochrome and easy to toggle, and you should have varied shades between elements, without too many colors that are "the same" next to each other or in close proximity. Another thing related to color I would recommend is using a lighter one for the background - the dark brownish-gray you used makes it so some of the elements, like the action lines, are lost in the color. Alternatively, you could make those lines white, but that'd take a bit more work to do, especially with the tone shading around it.
This last tip is more of something that might help you while coloring, it's not something you have to do but it helps out quite a bit for me - use multiple layers, folders, and clipping to layers below it. If I were doing this pic, I'd have folders for the guy, the girl, and the background objects (walls, bed, the shirt hanging up, etc.) Probably place the guy on top, start with a base layer of his skin tone, then clip all the other layers in that folder, related to him, to that base layer. That'll make it so when you color his shirt, you can go wild with it and it won't leak past that skintone layer you put down first. Same thing with the girl - and with her, since she's a bit more complex, you can add subfolders there working the same way off that main folder. Something to play with and learn, if you're game, and it could help you out.
Anywho, good start I would say - definitely better than I did when I started. I hope this helped out a bit! When you get a bit more confident, you can try shading using their stuff as a guide.
I use Photoshop. Yes my edge work needs lots of touching up. I correct my edges a lot but they're are mistakes I see only once it's uploaded. The difficulty also depends on the artist (Mariano really ain't easy)
I also use a ton of layer and folders when coloring. When I color it usually starts by making a new layer andclicking the girl's skin with the magic wand and adjust the selection afters and then i put that layer on multiply, it also saves a bunch of time when I want to do an demon girl alt.
And yes, the background color is always an afterthought(most of the time I have no idea what color I should use) and I sometime make it too vibrant and you lose focus on the action. I often tried to put an effect to give the image more punch but alas.
I don't know much theory so I will try to use your advices, might download the program you suggested
If the results are good I will most likely improve and reupload most of them : )