My first red velvet cake attempt didnt go very well.two things went wrong and i have no idea why?
Q
1. The cake sunk in the middle
2. The cocoa and the red colour separeted,in the middle the cake was red and the edges were chocolate brown.the colours didnt mix. I used liquid crimson red colour.

Hard to tell unless you post what recipe you used.

i used recipe posted in recipes by brandy7909,..Red Velvet Cake and icing.
I didnt get to the icing though was disappointed with the results.

I am assuming you made a scratch recipe? How old is your baking soda? Did it froth really well when you added your vinegar?

If you use too much cocoa powder and you don't have it well sifted into your flour it can separate in your baked cake. I always sift my cocoa (or at least run a whisk thru it), then measure out my 2 tablespoons, then sift it into my flour, then sift the flour cocoa together.

I also only use Dutch processed cocoa, not regular cocoa like Hershey's. Some claim it makes no difference but I disagree. I get much better results and a more uniform texture using Dutch processed.

Hope this helps!

I make a paste mixing my cocoa and red food coloring together and then add it to my wet ingredients right before adding my dry ingredients.

As for the leavening, make sure to add the baking soda and vinegar mixture immediately and mix only enough to make sure it is well incorporated. You must use the batter immediately.