Your troops continually consume food. Advanced soldiers need more food. When your consumption is more than your crop yield, your food will blink red. Your troops won't die even if your food stores reach 0, but you won't be able to upgrade buildings, research tech or train troops! Remember to always keep your yield higher than your troop maintenance costs! Build more farms and upgrade them, research food production, learn food production in Hero Skill Tree, wear Gear with food production bonus. Those will help you increase food production.