Write the noun that is the simple subject.

1/ The red color of my sweater looks good with the bracelet.
Answer: color.

2/ Some members of my family also enjoy stringing beads.
Answer: family.

3/ This beaded necklace is one of my favorite pieces of jewelry.
Answer: necklace.

4/ A friend of mine made it for me.
Answer: mine.

5/ The colors of the beads go together very well.
Answer: colors

6/ Markers of baskets use tools such knives.
Answer: markers.
Please , help me.

2 and 4 are wrong. The rest are right.

Subjects are never in prepositional phrases.

http://www.chompchomp.com/terms/prepositionalphrase.htm

Corrected responses:

1/ The red color of my sweater looks good with the bracelet.
Answer: color.

2/ Some members of my family also enjoy stringing beads.
Answer: members.

3/ This beaded necklace is one of my favorite pieces of jewelry.
Answer: necklace.

4/ A friend of mine made it for me.
Answer: friend.

5/ The colors of the beads go together very well.
Answer: colors.

6/ Makers of baskets use tools such as knives.
Answer: makers.

To identify the simple subject in a sentence, you can follow these steps:

1. Identify the verb in the sentence. The subject is usually the noun or pronoun that performs the action or is described by the verb.

1/ The red color of my sweater looks good with the bracelet.
The verb in this sentence is "looks." It describes the action of something looking good. The subject that performs this action is "color."

2/ Some members of my family also enjoy stringing beads.
The verb in this sentence is "enjoy." It describes the action of enjoying. The subject that performs this action is "members."

3/ This beaded necklace is one of my favorite pieces of jewelry.
The verb in this sentence is "is." It describes the state of being. The subject that is being described is "necklace."

4/ A friend of mine made it for me.
The verb in this sentence is "made." It describes the action of making. The subject that performs this action is "friend."

5/ The colors of the beads go together very well.
The verb in this sentence is "go." It describes the action of going together. The subject that performs this action is "colors."

6/ Markers of baskets use tools such as knives.
The verb in this sentence is "use." It describes the action of using. The subject that performs this action is "markers."