Do not use fluorescent colors, purple, pastels, bright, or modern-looking colors. Keep black to a minimum. Shirts and shifts can be natural for the lower middle class and white for the upper middle class. Trims were simple, such as ribbon and embroidery, and always colored - gold and silver metallic threads or trim were far too expensive as was lace. As one moves up in the middle class, trims can become more ornate and include the gold, silver, and lace for those able to afford the sumptuary tax required.
Jewelry also varies as one moves up in the middle class.
The lower middle class would have little if any jewelry and it would be simple in design. The upper middle class would wear more ornate jewelry. Belts with pouches, eating knives, tankards, etc. were also included. For the lower middle class, your costume should include the same basic garments as described for peasants, but they would be cleaner, neater, and newer. The shirt or shift could have a neck ruffle on the collar and the jerkin or doublet would be more fitted and perhaps made of the same material as the breeches.