Question Low Poly Water - flat shading triangles [ShaderGraph Image]

Could anyone help me out with the shader :
should be done in vertex - you can take the vertex position in object space and convert to uv (+ float3(0,5,0.5))
then use sine and from wave phase - Time*Wave Speed and take a noise to calculate new displacement on y.

After that, you need to calculate light you need normals - for flat shading squares you would use cross(ddy(worldPosition), ddx(worldPosition)). Is there is a way to get normals but in triangles?
Here is what I've got right now, as you see it interpolates color between the vertices. Is there is a way to not interpolate color that is taken from vertex in shadergraph?