Redness and swelling in the middle of a dog's toes may be caused by a variety of reasons, common ones include:

Interdigital inflammation: This is a common inflammation that may be caused by bacterial or fungal infection, It is caused by parasites (such as mites), allergic reactions, or foreign objects (such as small stones, thorns, etc.) entering between the toes.

Trauma: Dogs may accidentally sprain or cut their toes during activities, resulting in Redness and swelling.

Toe pad inflammation: Injury or infection of the toe pads can also cause redness and swelling.

Arthritis: Arthritis may cause joint swelling, which may also appear as redness and swelling if it occurs near the toes.