The colour on the new fence panels is unlikely to match the colour of the old panels even when treated with the same product. Unfortunately, this is the nature of wood. Every piece of wood is affected by type, age and condition and will respond differently when a wood finishing product such as a coloured wood preservative is applied. If the older panels appear to be a lighter colour than the new treated ones, an additional coat of preservative can be applied to the older panels to strengthen the colour and achieve a better colour match to the new or vice versa. The colour of newly applied preservative will settle down after a couple of weeks of weathering.