Try a mixture of about 1 teaspoon of baking soda in 1/4 cup of water. Mix it, put it in a spray bottle (like you would use for spraying perfume). Spray the side of the wood you are going to engrave and let it dry. When you engrave that, it will burn much darker, so you will likely need to increase your speed, or reduce the power when using that method.
Otherwise, I would suggest lowering the speed to get a darker burn and more contrast.