You could use product attribute for product variants such as Color, Size etc. In Catalog -> Attribute, you could add a product attribute to be shared by all the products. In "Add a New Attribute" form, fill all the field and click "Add" button. Here are some explanations to the fields.

• Attribute Name: the name that you assign to the attribute. E.g. Color, Size
• Required: if checked customer has to choose a value for this attribute before he could add the product to the shopping cart.
• Selection Type: only "Single Selection List" is available now. More types such as multi-select will be added in the future.
• Display Control: how different choices of the attribute are shown.
• Description: a brief description of the attribute