3.Pessimists warn us not ( ) for pretty colors, since the car will come in gray.

解析:考生应特别注意在主动语态中,不定式在let, make , have(使,让), see, hear 等词后面作宾语补足语时,to 应省略,但在被动语态中to 应保留。1.The workers asked the factory owner to let him go.2.The children were made to repeat their parents’ words for several times.