# How do I make the \$ i \$ blue variable?

``````a = {m - * s * i + [Gamma]* i - d * s, [Beta]* if - [Gamma]* i -d * i}
b = {s, i}
MatrixForm[D[a, {b}]]
``````

When I type this code in Mathematica, followed by

``````Solve[M[M[m-[m-[Beta]* s * i + [Gamma]* i - of * s == 0, i = 0]
``````

$$i$$ turns the color black and I start getting errors. Then I am unable to change the black now $$i$$ in the blue again. How do I change $$i$$ in the color blue?