First, I'm sorry but I don't know where "Front Royal" is. Most members of this board live in the US and we try very to stay within the law. In the US unless you own a registered machinegun conversion device or have registered your Omega SD (barreled receiver as an "upper" is not a HK roller locked term) as a SBR it is not legal in the US to possess a SD barreled receiver (less than 16" barrel) and a stock for said firearm whether you install the stock on the receiver or not. Please clarify.
As far as your finish question, even HK factory receivers and stocks can vary in color depending on when they were made. If you want the barreled receiver and stock to match, you can do every thing from buying a can of spray paint from the local hardware store and painting both yourself to sending them both to a shop that specializes in firearm finish. You get what you pay for. Spray can paint will not be as durable and solvent resistant as a professional firearms finish. I had Joe at Dakota Tactical refinish a converted HK 94 for me. What a fantastic job he did. It was $300 for the refinish with reparkerizing the internals. So can you live with the difference or spend the time and/or money. That part is up to you.
Scott