50 / 55 : stack plot colors

* [12645df](https://github.com/kangwonlee/nmisp/commit/12645dfbde370ed84c599c993119854b627898d8)
``` python
ylabels = ['s', 'i', 'r']

y = [sol.y[1], sol.y[0], sol.y[2]]

plt.clf()
plt.stackplot(sol.t, y, color=['orange', 'blue', 'green'])
plt.xlabel('$t(sec)$');

```
seems all orange