Why are the leaves purple??

Hello everyone!

I have this plant whose leaves just started turning purple and I do not know why....I mean, I have read on the internet about potential causes, it might be due to minerals deficiency such as phosphorus or maybe a disease....I am not sure which one it is and how should I act.

I shall attach some pictures to this thread so you can see the problem.

Description:

The leaves are still quite strong on the plant;

It is first the midrib and the veins that turn reddish purple, and then the rest of the leaf;

In comparison with the completely green leaves, the underneath of the purple leaves is yellower;

There are newly formed leaves whose midrib and veins have already turned purple;

I couldn't feel any difference between the texture of the 2 types of leaves;

The petiole (leaf stalk) of the purple leaves is quite dark (reddish purple) compared to the other leaves whose petiole is greenish/brownish-white;

The lower stem of the plant is light brown....I would say that it starts to become quite like a thin tree branch (looks quite keratinised)....which I think it is my fault as that portion also had leaves, but then they turned purple and I got them out, hoping that would solve the problem, but then that portion didn't regrew leaves and the top of the plant's leaves started turning purple as well;

The stems that lead to the branches that have quite a lot of purple leaves are dark as well;

I didn't see any small insects or other plants growing in it's pot, neither dry leaves, though some leaves look yellowish (neither green or pruple) and seem weaker than the other leaves but they are still strongly attached to the stem;

At the moment it only has one flower, even though I saw buds, some of them wilted.

I am not sure what my plant is (I am not really a gardener but I received this one as a gift and I really dont want it to die, so please please help!), but I have a hunch it is a catus dahlia, however I am not 100% sure on this.

What should I do? Should I buy any products for my plant? Should I take the purple leaves out? Please guide me!

Here are the pictures:

1.The purple leaves

2.The underneath of the purple leaves

3. The flower of my plant (for plant identification)

4.Comparison between those weaker (yellower) leaves and the greener ones

Thank you very much in advance for all of your help and advice, or even criticising :-) !