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?