MacBook Air doesn't start - no power on
So I tried SMC reset again and the light color changed to green for a moment and again changed to amber. But it still didn't turn on.


The amber light simply means it's charging.  Green means that it has detected the "one wire circuit" and is passing current to the MacBook.  It doesn't necessarily mean the battery is charged (I have a white 2007 MacBook on my desk with no battery and a green light on the charger)


  MacBook Air not starting after SMC reset. Earlier, my Mac was not starting after SMC reset or NVRAM reset.


You're doing a lot of resets.  Resetting the SMC and NVRAM should be rare:


SMC - power, battery and fan issues.  For example, you're having issues charging your Mac or the fans are running at full speed or it doesn't wake from sleep when opening the lid.
NVRAM - preboot environment variables.  You reset this when you're having trouble booting, or the initial resolution is incorrect, or you're having issues with the startup volume (boot).



  Nothing was happening on SMC reset. Battery Status was Service Required.


So, there was an icon that said "Service Battery."  An SMC reset isn't going to fix a bad battery, but it won't hurt.  

From Apple; Check the condition of your Mac computer’s battery




  Service Battery: The battery isn’t functioning normally, and you may or may not notice a change in its behavior or the amount of charge it holds. Take your computer in for service. You can continue to use your battery before it’s checked without harming your computer.


There are any number of things that can cause a Mac to not turn on from the power button being faulty to an issue with the logic board.  Unfortunately, you'll have to take it in for service when you can.