What is the official dress code given by the NTA for January? Are jackets, hair pins etc. allowed?

Yes, you can wear full sleeve jackets but too simple not a fancy one. Avoid wearing clothes having too many pockets, fancy stoles, scarfs etc. Wear loose and comfortable clothes. Don't wear any metallic items or any clothes containing metal.
http://engineering.careers360.com/articles/jee-main-dress-code

Dear aspirant,

The thing is there is no particular dress code that you must follow. But under any circumstances, you should follow these few criteria . As it's winter time you can wear jacket but it would be better if you keep it simple. But for your betterment you shall follow these dress codes. The,

• Candidates shall not wear any cloth consisting of metallic items. Metallic items causes problems during frisking and must be avoided

• Candidates are not allowed to wear cap/ muffler or any cloth over their head

• Students must wear light-weight comfortable clothes to avoid discomfort during writing the exam.

• Wearing any kind of jewellery or ornaments is strictly prohibited

• Candidates must not wear goggles, rings, bracelets or any of such items

Hope this helps and all the best!