When I send a text in iMessage sometimes some of the writing goes orange and the rest is normal black. Its not certain words that change to orange its really random and doesn't happen with every iMessage. Have looked in settings but cant find anything I have changed or selected. I am using iOS 10.2 on an iPhone 7.
That's because those words can be replaced by Emojis. Apple released that with iOS 10 and when you type something in iMessage and select emoji keyborad(if that has a meaning matching with Emojis) it will make that word orange. If you want to replace that word with an emoji, then you simply have to touch that orange coloured word and it will be replaced by that emoji.
It is not a problem with your phone or any software issue. It is a feature introduced by Apple.
Ex: Type My car is down. Then select emoji keybord and it will show "car" word orange coloured.