Add new number to existing iPhone contact, replace the old number
The blue number (the bottom one in your screen capture) is the one which made the call. The same works for emails in contacts that have more than one.

Here for example, I have a contact with a landline (home) and a mobile phone.
Incoming and outgoing calls to the different numbers of the contact are highlighted in blue.

This is a screenshot of a call made to the mobile number of the contact (not an iPhone, its my grandma):



This is a screenshot of a call made to the landline:



Also, if you missed the call, the number calling will turn red instead of blue.

This is a screenshot of a missed call from the landline: