If it's a PVC patch you've ordered from RIBstore then you can use it either way up. Both the shinier and more matt side are made of the same PVC, so you can use the patch either way up to best match your inflatable.
However, if it's a hypalon patch, then this isn't the case. One side is the hypalon CSM layer which gives good resistance to weather, chemicals and abrasion. This side is also the 'coloured' side. The opposite side is neoprene to ensure high glue-ability and air tightness.
The back of the fabric is typically a smooth dark grey or small stippled dark grey finish, although this is a small stippled black finish if you have Anthracite Fabric Impression material.
Military Grey is probably the most asked about in regards to which way up. The answer is the lighter side is Military Grey and the darker side is the back - as shown in the photo.