A red tip of the tongue is very common. The tip corresponds to the Heart and the Shen and a redness of the tip always indicates emotional stress affecting the Heart: the redder the tip, the more intense the stress.
Why is the tip affected and why does it become red? The Heart is affected by all emotions as it houses the Shen that feels them. When we feel angry, anger affects the Liver automatically, but it is the Shen of the Heart that “feels” the anger. Thus, a red tip of the tongue may be caused by any of the emotions, e.g. anger, sadness, grief, worry, fear, etc.
Why does the tip become red? The first effect of emotional stress is some Qi stagnation: when Qi stagnates, it frequently gives rise to Heat and hence the redness of the tip. If, besides being red, the tip is also swollen and has red points, it indicates that the emotional stress is even greater.