I have a alpha imperial flame and a toxic flame. I want to change the colour of the toxic one, would I need 2 particle textures

No.

1 flame particle texture applies to all the flames you have activated.
Activating 2 flame particle textures wouldn't do anything.

You have to find out yourself as to what image/combination of colors would emit the desired color.
Or you can just request a flame particle texture.

Note: Please don't make duplicate threads in the future. Thanks.

How come I can't put a flame in the market

Told you in-game, but whatever.

Click on the icon of the flame, where you'll come to the "update price" page.

Yeah, I posted it then asked you ingame

I'm also pretty sure you can only change the color of void flames.

Wrong. Anyways, it's pretty easy, since the dark flames colors (if it isn't a blending one) uses almost all the rgb scale, and alpha imperial is pretty blue-black, you can manage it pretty easily.

For void flames, if you want a green flame, just set a red particle flame. In your case you will need to do something similar, but with some limitations.

I am not as good at art, but it's easy when you know something of colors coming from physics in the univ ;D

Nope.

To set the desired color for a void flame, you simply invert the desired color, and use the final product as your flame particle texture.