When I send a message from my iphone it's green or blue?

Hi. When I send a message to my friend an we both have iPhones it is blue speech bubbles but when I send it to a friend who doesn't have an iPhone it is green. Is there a way I can make the green bubble blue even though they don't have an iPhone?

Its blue because its iMessage service… Its free of cost… You can even share photos via iMessage
You can't change its color.

No.

The green bubbles are regular text messages
The blue bubbles are iMessages

They are differently coloured in order to tell which is being used.