Will it be problem to use wd green as primary hdd

they are for whatever you want them for... it's like comparing a ford fiesta, mondeo and mustang, between the green, blue and black

Here's what WD says about their intended use:
Green:
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/287...

Black:
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/287...

how is reliability among black,blue,green?

not a factor IMO - it comes down to cost and amount of space you want

The greens have a 90 day warranty and are considerably slower. The blue & black are both faster but the blue has a 2 year warranty where the black has a 5 year warranty. FWIW, I've been using WD exclusively for over 15 years and have yet to lose a bit of data.

I currently use a WD caviar green 2tb as my primary drive due to failed SSD and it isn't any noticeably slower than any other platter drive for normal use. Maybe only a tiny bit in load times in games. Check the benchmarks and you will see that the WD caviar green drives end up very close to caviar blue in read and write speeds.

thanks everyone for help

@ram it looks like green and blue both have same warranty period

BTW will it decrease the lifespan of hdd if i use green hdd as primary drive?

I'm glad you mentioned the warranty period. I looked it up right before I posted that but I was looking at refurbs. I thought it seemed short. Your warranty won't be affected by use as a boot drive.