This is acrylics, using the Interactive Atelier range. Using just 2 colours (yellow, brilliant violet) and black and white was fun in this wave painting because it made my colour choices much simpler.
You'll note that it looks like there is actually blue used in the painting but that's simply a neutral gray appearing cool when surrounded by warm colours. Nifty! Give it a try.
Learn more about painting waves in my new course:
Learn more here: https://mypaintingclub.com/lessons/232-Painting-Waves