Lava lamps have a simple build, there are even tutorials on the web that are showing how to make a lava lamp at home.
It is better to see them live before ordering one from the Internet. The videos from the web might not help that much.
The lava can be either green, red or purple but the light source is the one that has a big impact. If it is yellow and the “lava” is green, it will give a funny look rather than something serious and cool.
You can replace the yellow light with a white one but these lights might be harder to find. So, the lava lamp is worth to buy or not?
You might get bored of it after you use it for a few days. Not to count that in the first few days, it will distract you.
The good thing about them is that they are cheap, around 20-30 euro for one, so the price should not be a real problem.
Regarding their safety, you have to keep the lamp placed on something that doesn’t heat, not on wood or on a carpet, only on something solid that won’t heat. These lamps do heat a lot after a few hours.
They can run for hours and hours without any issues but I would recommend to keep them “on” only while you are awake and in the room with the lamp.