Manage products with multiple sizes and colours

I am looking to build a shop which sells clothing, and my client needs to be able to add a range of colours and sizes to each product.
Some of the products for example might be available in S, M, and L but other products could be available in 10, 12, 16 and 18.
This is the same for the colours.
What is the best way to build this from an admin point of view?

I have built one similar to this but that had a products table, a size table and a colour table, but you had to manually enter each size and colour for each product (so the colour table would have multiple entries for 'Red' for example - one for product 1 and one for product 2).

Could I have a colour and size table which would have all the possible variations, then have for example 10x size fields and 10x colour fields within the product table. These coud be populated from dropdowns fed from the relevant tables.

Any help would be greatly appreciated.

Regards,

Lee