I have made marble cakes in the past & they are nice & moist & have great flavour. The problem? I want the different colours to be more distinct. I make the mix, place white on the bottom, then add dollops (don't you love that word?) of chocolate & red cake batter & swirl with a spatula. When it is baked, it just looks like one colour cake. What am I doing wrong?

I swirl the dollops with a skewer. You can go through them in one direction and then go thru them once more at a 90' difference. It just takes practice and less passes gives a more distinct bold look. Your dollops should be equally surrounded by the white batter. I use 3/4 of the batter white in the pan and the last 1/4 reserved to make chocolate to drop the dollops with.
The more you pass through it the more marbled or zebra like the pattern will become.