Wow this reminds me of a red silk scarf that stained my old nylon flight jacket from the Air Force. I had it hanging in my apartment as some wall art for years and the dye bled. Not cool! Maybe washing the stock Alfa cover once when you get it is a good idea?
Now for outdoor covers I can swear by them. Cover King is good. Try JC Whittney online or Amazon. There are all kinds of levels and quality you can get. I have a breathable gray cover with fleece lining for my Ferrari. It is very good but was fitted for the 348 with square styling so it is a little bloused at the corners now and not quite large enough. It leaves behind fuzzy lint so an extra towel on the soft top helps. Fine on paint.
On my Miata, I had it covered for 6 years downtown. Made special tags and flaps for my resident stickers over the years. It kept my paint nearly mint for years and I got many compliments over that time. Still do. I started with a simple breathable cheap waffle style. They come with grommets and I used lock cables for them. This presented headaches over time since high wind and curious people would pull on the cover and parachute it around. I found 2 bungee cables locked together at one end, with a belay line tied to the other was the best arrangement. This way you could throw the cord under the car, and the belay line would be easy to reach BEFORE stretching the bungee. But you want it to stretch and hold tension, so you grab the hook on the end of the bungee by pulling the belay line out to you and then place the hook in the grommet on the cover. Again you have to be careful to save the paint. You may end up with some scratches or at worst gouges in the sills, but overall it is worth it. I think you can cover the hooks with tubing for more protection. Shoulda' thought of that sooner

Also had a silver nylon one but it didn't fit as well. You can layer them, nylon all weather on top, breathable garage queen underneath
Never cover a dirty car. Be careful when wet or iced over. Luckily, in one ice storm I was able to pry it off slowly with no damage to the car. It will help the paint and the interior. Way better than nothing!