I am short on my red velvet box mixes and they are not redily available to me, what can I use to extend the one box mix???

I've used one red velvet cake mix combined with one German chocolate cake mix. Then I added a little more red coloring to keep that deep red color. I think it even tasted better with the added chocolate flavor.

What sort of red colour did you use?? wilton gel or the grocery store liquid stuff??

I added some Wilton no taste red. I can't remember how much....I think I just eyeballed it. It worked very well, and I didn't have to worry about it adding an odd taste to the cake. HTH

If you haven't already baked .... why not use my WASC cake recipe?
It's perfect for any cake flavor and give you the same amount as if you used 1 & 1/2 boxes of mix.
The other thing..... you don't need to add extra color when you mix it w/a choco cake....there is *FAR MORE* than enough red coloring in those mixes already!

I guess I should clarify.....I used a doctored cake mix recipe when I combined the 2 mixes. A doctored recipe makes the cake texture much better and it will also extent the batter a bit. There probably is enough red coloring in the mixes like kakeladi said, but my bride really stressed that she wanted the color to be red. Therefore, I added extra coloring just to make sure she was happy. WASC will also work very well. Lucky you....you have lots of choices!