Fatigue

Basically, you have sent out your troops onto too many sorties and they are now exhausted. Swords sent on sorties get exhausted faster if they do not attack as much. The background colour represents the level of fatigue that the swords could be at, ranging from Red State (worst condition) to Sparkling/Sakura Petals State (best condition). Red < Orange < Normal < Sakura Petals Warning! If you send your swords out to battle when they are orange or red, they run the risk of breaking even when they only suffer from medium damage. If your sword is orange or red, it is best to return to base as soon as possible.
 * How to Recover from Fatigue: 
 * The said swords recover naturally over time, as long as you do not send the said swords into battle.
 * Alternatively, if you have purchased the dango item from the purchase shop, you can use it to help your swords instantly recover from fatigue by clicking the green button at the very bottom of your party selection page marked 疲労回復. Be advised, however, that the item feeds all the swords in the selected party, meaning it is better to save it for a later instance when more than just a few swords are fatigued at the same time.
 * How to Achieve Sparkling state: With the said sword being an MVP of at least 2 battles, it will achieve the Sparkling state. Your swords tend to do perform poorer in battles as their fatigue level worsen, so be sure to not overwork them too much.