A horse of a different color?

i was igned a project on the idiom " a horse of a different color " i don't know exactly what it means. is it to stand out? or to be different? not fitting in? and one more question, i need it in a conversation. so like- "i was nervous as i stepped on stage" "go break a leg" yelled the director as i walked onto stage. something like that. thanks a bunch!!!!!!!