When you don't have enough mana to cast the currently selected spell, your mana turns like a brownish-red and the extra amount that you will need to cast it turns, I guess, a dark or faint brownish-red. That extra amount is difficult to see (at least for me), especially against certain backgrounds. In fact I had played the game for many hours before I even noticed it was there, and for some time thereafter I wasn't entirely convinced that I wasn't imagining it.
Could it please be changed to make it more obvious? Perhaps something like this mock-up: