Q: When using iMessaging on an iPhone6, why does the same contact show up as blue (message will deliver via iMessage) when I send to her email but greeen (message will go as SMS) when I choose to send to her mobile phone number?

Possibly because her phone number is not associated with iMessage, but her email is?

crfruns wrote:

 

Thanks. Yes, that is correct but why does it happen this way?  I'd like both email and mobile phone to be associated with iMessages.   How does one associate their contact info with iMessages? And iIs this something I do or something my contact must do?

the issue is wither her phone and not yours.

She will need associate her cell with imessage

Link your iPhone number and Apple ID to use iMessage and FaceTime - Apple Support

Its not up to you. If the contact has not associated there number with iMessage you cannot use it to send an iMessage.  They need to have an iPhone to be able to do that though.

crfruns wrote:

 

Great, this answers my question.  Thank you!

You're welcome.