I really like greeenpro's Blood-gushing Candle
but I couldn't find the microwave wax that he used. So I found another way to make a white candle with red wax down the middle.
Step 1: First melt a hole down the middle
Heat up a metal rod and use it to melt a hole down the middle next to the wick starting at the bottom of the candle.. You will have to keep heating the rod again several times to get right to the end. I found the easiest way was to hold the candle in one hand and the hot rod in the other. Be careful not to go too far and melt a hole right though the end. Hold the candle at an angle so that the melted wax can run out. You may want to catch the wax that runs out in a tin so it doesn't go all over your work-bench.
Step 2: Pour red wax into the hole.
Melt some red wax in an old tin and pour it into the hole. I found it was best to just pour it in small amounts rather than all at once. Don't get your wax too hot or it will melt a hole in the side of the candle and all run out