In an exhibition there are $100$ paintings each of which is made with exactly $k$ colors. Find the minimum possible value of $k$ if any $20$ paintings have a common color but there is no color that is used in all paintings.
To solve the problem, we need to determine the minimum number of colors \( k \) such that:
1. Any 20 paintings have at least one common color.
2. No single color is used in all 100 paintings.

We will use a combinatorial approach to prove the general statement and then apply it to our specific problem.

### General Statement:
Let \( n \) be the minimum number of colors needed to color \( m \) pictures such that:
1. Any \( n \) pictures have a common color.
2. No single color is used in all \( m \) pictures.

### Base Case:
For \( n = 2 \):
- If \( k = 1 \), then the condition that no single color is used in all \( m \) pictures would be violated.
- Therefore, \( k = 2 \) is the minimum number of colors needed. We can color the pictures using the set \(\{c_1c_2, c_2c_3, c_3c_1\}\), ensuring that every element is used at least once.

### Inductive Step:
Assume the conditions hold for \( n = 3, 4, \ldots, l-1 \). Now, we will prove for \( n = l \).

1. **Achievability for \( n = l \)**:
   - Let \( S \) be a set such that \( S = \{c_1 \cdots c_{l}, c_2 \cdots c_{l+1}, \ldots, c_{l+1} \cdots c_{l-1}\} \).
   - Each element of \( S \) contains \( l \) colors, taken cyclically.
   - Any \( l \) elements chosen from \( S \) will have a total of \( l \) missing colors, but since we have used \( l+1 \) colors, there will be a single common color.
   - If all \( l+1 \) elements are taken, there will be no common color.
   - Using every element of \( S \) at least once to color all \( m \) pictures will fulfill the conditions.

2. **Minimality of \( l \)**:
   - Assume \( q \) is the minimum number of colors needed to color \( m \) pictures such that there is a common color in every \( n \) pictures, but no common color in all \( m \) pictures, where \( q < l \).
   - From the inductive hypothesis, \( P(q) \) is true.
   - Consider a set of \( q \) pictures; they have a common color \( c_j \).
   - In the remaining set of pictures, there must be at least one picture that does not contain \( c_j \), otherwise condition (ii) would be violated.
   - Adding that picture to the initial set results in \( q+1 \) pictures with no common color, leading to a set of \( l \) pictures with no common color, contradicting the initial assumption.
   - Therefore, \( l \) is indeed the minimum number of colors needed.

### Application to the Problem:
Given \( n = 20 \) and \( m = 100 \), we need to find the minimum \( k \).

1. **Achievability**:
   - Using the general statement, we need \( k = 21 \) colors to ensure that any 20 paintings have a common color, but no single color is used in all 100 paintings.

2. **Minimality**:
   - If \( k < 21 \), then by the inductive step, we would find a contradiction as shown in the general proof.

Thus, the minimum possible value of \( k \) is 21.

The final answer is \( \boxed{21} \).