This is your original image
And this is my paint-over (I accidentally painted a bit red on Godzilla's head, ignore that part)
First of all, you shouldn't use pure black. Something is only black when it absorbs all light (or if there's no light, in which case everything will be black). In reality, most of things we call "black" has a certain level of gloss, which makes then reflect light. Therefore, they will take the color of the light. If it's white light, they would appear dark grey to light grey, but not pure black. If it's light reflected from other objects, they will have some of that color as well. For example, placing fire near a black object will give it some yellow-orange color. Pure black is for things that are very glossy, like metal.
Same goes for pure white. There's almost nothing that looks pure white, except for metal or wet objects.
Now, about the values (how light/dark something is). The sky and the water surface in your picture is too bright for a night scene, so I lowered their values a bit, plus it gives a darker atmosphere. I noticed that Godzilla is holding a flaming jet in his hand, but the value is so dark it's hard to see anything other than his claws. Furthermore, fire is a light source, so things around it should be brighter. I brightened up the area around his hand, so that it would have enough contrast to be more visible against the dark background, and give the impression that it's a light source. I did the same for the fire in the city and the full moon, since they are usually very bright. About the vapor/fire thing in his mouth (not sure which), I add some lines of lighter value, so it will act as either value variation for the vapor or glow for fire. Also note that blood mix well with water, so it won't have a sharp edge.
Next is color. From what I've seen, you use a lot of colors with very high saturation. For colors like red and yellow, it doesn't look very natural. For example, the city sky in the Godzilla picture, you used a very strong red. In reality, it looks more like dark brown with some red tints.
Same goes for hair: blonde is more light brown than yellow, red hair is more brown red than just red.
Lastly, the fog/smoke thing that you often do. Don't just add dark/light grey brush strokes over them, they make the picture looks messy. Instead, use similar colors with some changes in hue/saturation/value...
...and give them more shape variation and make them look more like a moving mass.
I hope this can help.