How can I improve my COLOR SENSE ?

Color sense comes with experience. Coloring an image consists of three parts: base color, shading and material effects. For the beginning you need not bother about material effects, those are tricks mostly anyway.

Base coloring gives a flavor to the image. Styles vary from grey-scale to the typical asian color shock. I personaly favor colors that surround a certain central color-scheme.

Shading is somewhat more tricky for it adds the depth component to your images. For the beginning i would advice always using a fixed light source (i.e. parallel light from the upper left), so you can focus on getting the shades right, rather than having to worry about light positioning.
I've done images with many light sources and other than additional difficulties for the artist it doesn't appear to add to the overall impression of the image.
Since you're into manga i would suggest using three color shading. That is one color for the parts facing the light, one facing away and one for everything else. Or you can try smooth shading with many different darkness levels.
To learn this i would suggest grabbing a very soft pencil (2b or softer) and start drawing shades in grey scale. Whatever you learn here you can use for colored images as well.
Once you're halfway secure in this you could try to shade the drawings you already uploaded. You may find that even the worst drawing looks several times better with a proper shading.

I hope this was halfway useful. I started off where you are now and i am certain that you will figure in time. Just never give up and observe what other artists do. Learn from their masterpieces and their mistakes.