You should be going for a milky look to your paint.
Marbled wood effect paint.
If you add too much water and you will lose the vibrant colors.
To water down your paint a bit add equal parts water to your acrylic paint and mix with a wooden skewer.
Blend out fine veins.
First you will need to marble the wood panel.
Then mix your paint in disposable cups.
Create the look of marble at a fraction of the cost with a faux finishing technique.
How to water marble with acrylic paint.
Choose your colors based on the type of marble you wish to recreate.
This technique is messy so i use scrap cardboard to cover my table.
Start with your plain wood panel and protect your work surface well.
Slowly drip the paint into your liquid starch mixture.
Dip a detail brush in diluted white or very light green paint and add fine lines to simulate veins.
When you re satisfied allow the paint to dry for several hours.
How to draw marble.
When the paint is dry add marble veining.
Remember to paint the veins over the edge of the tabletop as well.
Use a dry blending brush to smooth out the fine veins.
For this i have an acrylic paint pouring technique that will give you a marbled look.
Repeat this step at least two times and add in some white veins to create highlighted areas.