In order to use a texture as the color, the texture data needs to be provided as a uniform to the shader and the color needs to be sampled from the texture on each fragment.
If the texture is already available when creating the shader such as when it is passed in to a shader material via a parameter, then you can use the textureSampler2d function to create a sampler. By passing in the texture directly to this function, the texture does not also need to be added as a uniform.