Find the smallest positive integer $n$ such that if we color in red $n$ arbitrary vertices of the cube , there will be a vertex of the cube which has the three vertices adjacent to it colored in red.
1. **Understanding the Problem:**
   We need to find the smallest positive integer \( n \) such that if we color \( n \) arbitrary vertices of a cube in red, there will be at least one vertex of the cube which has all three of its adjacent vertices colored in red.

2. **Analyzing the Cube:**
   A cube has 8 vertices and each vertex is connected to 3 other vertices. We need to ensure that at least one vertex has all its neighbors colored red.

3. **Considering \( n \leq 4 \):**
   If \( n \leq 4 \), we can color the vertices in such a way that no vertex has all three of its neighbors colored red. For example, we can color any 4 vertices on one face of the cube. In this case, no vertex on the opposite face will have all its neighbors colored red, and no vertex on the same face will have all its neighbors colored red either.

4. **Considering \( n = 5 \):**
   We claim that if \( n = 5 \), there will always be a vertex with all its neighbors colored red. To prove this, we use the Pigeonhole Principle.

5. **Applying the Pigeonhole Principle:**
   The cube can be divided into two sets of 4 vertices each, corresponding to the "top" and "bottom" faces of the cube. If we color 5 vertices, by the Pigeonhole Principle, at least one of these sets must contain at least 3 red vertices.

6. **Case Analysis:**
   - **Case 1: One face has exactly 3 red vertices.**
     - Without loss of generality, assume the "top" face has 3 red vertices. The possible configurations of these 3 vertices will always leave at least one vertex on the "top" face with all its neighbors colored red.
   - **Case 2: One face has exactly 4 red vertices.**
     - If the "top" face has 4 red vertices, then any vertex on the "top" face will have all its neighbors colored red.

7. **Conclusion:**
   Since in both cases (3 or 4 red vertices on one face), there will always be a vertex with all its neighbors colored red, we conclude that \( n = 5 \) is the smallest number satisfying the condition.

The final answer is \(\boxed{5}\).