Return an object but with one property different
As said in a comment above, using a conditional assignment would be your best bet:

class Project {

  static createProject(data) {
    return {
      owner: {
        name: data.owner,
        avatar: data.ownerAvatar
      },
      users: [
        {
          name: data.owner,
          avatar: data.ownerAvatar,
          active: true,
        }],
      name: data.name,
      retrospectives: [],
      color: data.name === 'default' ? '#1F2532' : pickColor([])
    }
  }
}