Why is the following not a probability model?
$$
\begin{array}{lc}
\text { Color } & \text { Probability } \\
\hline \text { Red } & 0.3 \\
\hline \text { Green } & -0.3 \\
\hline \text { Blue } & 0.2 \\
\hline \text { Brown } & 0.4 \\
\hline \text { Yellow } & 0.2 \\
\hline \text { Orange } & 0.2 \\
\hline
\end{array}
$$