the real problem is probably the type of cocoa you are using. I'm assuming you're in the uk. For red velvet you need a paler cocoa- basically you need natural cocoa NOT dutch process. Its a specialist product , some health food shops may sell otherwise its the internet.
Panda is bang on. Silver Spoon used to do a colour called 'red'. They subtly changed it to 'natural red' about 18 months ago and it makes absolutely no difference to the colour of anything. Even putting it in frosting makes it go brown.
Daisy is right in that getting the gel food colourings is the way to go I think. You can pick them up in Asda or (at a push) Hobbycraft. The ratio is slightly different but it will definitely make them red.
the gel food colourings brought in specialist cake hops are usually strong enough to colour anything. I made ds1 a royal blue chocolate cake with the gel food colouring, was a bit darker than id hoped as was for his football teams colours, but was still better than the attempt with food colouring brought in the supermarket.
I have triend making red velvet cupcakes a couple of times with my neice now but everytime we make them the cocoa powder overpowers the red food dye, turning them brown everytime, even when use a small amount of cocoa & more food dye (the mix was even a nice red colour)! Does anybody know how to stop this & keep the cupcakes red in the middle? So confused! Pls help !!