I am doing an E-commerce site like Ebay but for clothing industry and I want a way that users can manage colours and sizes.
Sizes should be based on quantity.
Based on this what,s the best way to show it on front-end.
Avoid drop downs if you can. See JohnLewis.com for a good solution to showing sizes and their availability.
For touch devices the use of + - as buttons can be effective.
Being experience eCommerce ux designer - I would like to recommend to throw some different kind of ui patterns to users to understand their feedback. Here is the patterns list; http://ui-patterns.com/patterns/ProductPage
Btw - I like your design but just make sure that labels should be prominent by improving the contrast.
my suggestions; Color - Either button or dropdown, Size - Dropdown, Qty - textbox