13. In a $3 \times 3 \times 3$ cubic glass frame, there are 27 unit cube slots. Each unit cube slot can hold at most one colored ball. To make the front view, top view, and left view as shown in the figure, how many colored balls are needed at least on the cubic frame? Please place them out.

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13. In a $3 \times 3 \times 3$ cubic glass frame, there are 27 unit cube slots. Each unit cube slot can hold at most one colored ball. To make the front view, top view, and left view as shown in the figure, how many colored balls are needed at least on the cubic frame? Please place them.

【Analysis】First, analyze that there are 9 colored balls in each of the three views, so the bottom layer must have 9 balls. Next, the placement method requires that the balls can be seen from two views.

【Solution】Solution: According to the problem:
First, place 9 colored balls on the bottom layer to satisfy the top view;
Next, place the balls in positions that can be seen in both the front view and the left view. Place 2 balls at each position along a diagonal to meet the conditions.
$$
9+6=15 \text { (balls) }
$$

Answer: A total of at least 15 colored balls are needed to meet the conditions.