Hi Sumistha,
You are sort of doing 2 things here - exploring designs through sketching and presentation rendering.
With your sketch explorations you were getting ahead and the bottle sketches are quite good, now focus on sketching more dynamically allowing you to create these efficiently while still communicating all the subtleties of the design.
With your presentation renderings I was trying to convey something very subtle - adding warm and cold tones in the product to give it more depth and realism.
I cannot find a great example at the moment but for example compare these two:
A quite good render, but a bit dull.
Note the subtle shading in the product surfaces reflecting colored areas in the environment. It adds much more life to the image.
These classic automotive tutorials are also great in showing what warm-cold contrast can do:
I think apart from the drop shadow your last headphone sketch was the best one.