I made a red velvet cake following the instructions very closely, but my cake turned out marbled. It looks like a chocolate cake marbled with red cake. Weird, right? It rose beautifully and am looking forward to checking out the texture when it cools completely. Any advice out there? Thanks!
The only thing I can think of is that you did not mix the colouring in all the way.
When are you adding the coloring? In the recipe I use I first mix the sugar and oil, then add the coloring. This allows the ability to really mix the coloring well before putting in the buttermilk and dry ingredients so that the mixture is not over beaten. Sounds like the color just did not get mixed in well.
Sift the cocoa and baking soda with the flour.
Mix the vinegar with the milk and vanilla. Mix in the red colouring if you are using gel or the grocery store bottles stuff.
Do these regardless of what the recipe says, they help the final cake to be properly mixed without overmixing.
I usually mix my cocoa with my red color and vanilla, then add that after incorporating the eggs. The flour and milk additions that follow ensure it's thoroughly mixed.