If you cast a spell on one of your friends to make the friendship stronger, to be their best friend would that be considered black magic, without permission? It almost reminds me of a black magic love spell on a certain person wanting them to love you, but instead, you want someone to be your best friend.
Attempting to alter someone's free will can bring Karma back on your Even if you believe you intent is good, trying to alter someone's free will can have consequences. What you put out into the universe you get back.
Firstly I do not believe magic has colors. It really depends on your intention wether the energy you send forth is positive or negative. However casting a spell against someone's personal will is not something I like or support. So I would recomend to try and sort things out with your friends and not use any spells that apparently you are ansure regarding their results.