
[A green beefsteak-shaped tomato with a sudden orange coloring on one side of it.]
It is ripening i.e. it has stopped growing in size. The ripening is the last and the fastest phase of a fruiting process. That is when the rotting-hastening sugars, and seeds form.
Usually, if you definitely want full seeds and/or you want the fruit to have maximum flavor, it is recommended (by me, not the consensus) to let the fruit age a bit. Not too long or it will be all soft mush i.e. half-ketchup already. Maybe pick it a day or two after it reaches peak redness. Please not that by then, it has already softened significantly.
If you want a crunchy ‘mato, pick it a day BEFORE it is ripe. Either way, the fresh consumption window is within mere days of ripening. That is why most of tomato production goes straight for manufacturing non-spoiling vinegar ketchup.
Those flowers in the background are dried up cucumber flowers. I like growing things wild, entangled and mixed up.
The cultivar AKA variety is most likely Marmande. It mostly affects the shape of the fruit, nothing else. E.g. my Moneymaker and Marmande plants are identical.
This ripening happened in a relatively cool room with no significant indoors heating sources nearby. This means that the common claim about tomatoes being heat sensitive is not very well substantiated. Tomato is considered to be a particularly needy plant.
It is winter outside. Seasons do not matter with indoors gardening. So feel free to express yourself freely with plants all year.