Why does my iPhone sometimes send texts to other iPhones as iMessages then regular texts to iPhones?

Like when I send a text to my friend that has an iphone, it usually sends as an iMessage then it will randomly send as just a regular text and it will be green and not blue.

Here is a good explanation at cnet: http://www.cnet.com/...are-green/

  • Why won't my iPhone send iMessages to other iPhones? I can use iMessage on my iphone4 with no problems at all to iPods, but not to my partners iphone4, it always defaults to SMS. Its frustrating, because we live in an area with poor mobile coverage, so if I need to message him urgently, iMessage in theory should be the best way of doing it. Does anyone know how I can alter this please?
  • Why on my iphone does it sometimes show texts in green, and sometimes in red? I have a iphone 4s, my son has iphone 5. Sometimes when i text him, the text appears in green, and sometimes it appears in blue. Does anyone know why this is?
  • Why is my Iphone not sending as Imessages and receiving iMessages from one person? I'm texting my wife and we both have iPhone's and she is getting the messages as an iMessage but the ones I'm sending her on my end show up at green. Sometime it will go as blue, get the little red exclamation point then send as regular SMS. I'm texting someone else too and she has an iPhone, and she is getting mine as regular iMessages and mine are sending as regular iMessages. Why aren't they sending to me wife as iMessages?
  • Why does the whatspp of my iphone look different then other whatsapp on iphones? I noticed that the whatsapp in my iphone is bit different from the other devices.like, my fonts are not as dark as other iphone fonts. They have neat pattern of send and recv txts.dark wallpapers background… Not only about the whatsapp. My fonts are not as deep as the other iphone device. I bought my phone from online site flipkart… Please do suggest me how can i change or darken my fonts