I mostly wear neutral colors and earth tones

You and a friend are talking about your tastes in fashion. You don't like wearing bright colors, so you describe your taste this way.

mostly (do something) 

"Mostly" is a good word for conversation and casual English. A good alternative in more formal English is "primarily": It has primarily spread through word of mouth.

neutral colors:   "Neutral colors" are white, black, and gray. Sometimes colors like brown are considered "neutral" as well if they're not bright. You talk about "neutral colors" when choosing things like clothes, accessories, curtains, furniture, and so on.

earth tones : "Earth tones" are colors that can be found in dirt, rocks, and trees. Earth tones include shades of brown, orange, green, dark red, and some shades of yellow.

People use the phrase "earth tones" to talk about clothes, makeup, paint, and home decorations.