Any Idea on how to assign shared options whilst creating a variant for a product using code?

Currently I have this which essentially creates the shared options, but once created I would like to assign certain options to a variant

if (existingProduct == null)
                            {
                                merchelloProduct = this.MerchelloServices.ProductService.CreateProductWithKey(product.Name, product.ProductCode, price);
                                merchelloProduct.Available = product.IsPublished;

                                var colour = string.Empty;
                                var size = string.Empty;
                                foreach (var variant in product.ProductVariants)
                                {
                                    foreach (var option in options)
                                    {
                                        if (variant.Colour != null && variant.Colour.Name != colour && option.Name == "Colour" && !option.Choices.Contains(variant.Colour.Sku))
                                        {
                                            options.First(x => x.Name == "Colour").Choices.Add(new ProductAttribute(variant.Colour.Name, variant.Colour.Sku));
                                            colour = variant.Colour.Name;
                                        }

                                        if (variant.Size != null && variant.Size.Name != size && option.Name == "Size" && !option.Choices.Contains(variant.Size.Name))
                                        {
                                            options.First(x => x.Name == "Size").Choices.Add(new ProductAttribute(variant.Size.Name, variant.Size.Name));
                                            size = variant.Size.Name;
                                        }
                                    }

                                    var merchelloVariant  = this.MerchelloServices.ProductVariantService.CreateProductVariantWithKey(merchelloProduct,)
                                }

                                this.MerchelloServices.ProductOptionService.Save(options);
                            }

if anyone could help that would be appriciated