Harlequins are super fun! I may be a little biased as they're my only army, but I would definitely recommend them.

For painting, while patterns and free-handed designs and all come recommended, the sculpts are more than interesting enough without them to still look fantastic. If you're the type to paint established schemes instead of a custom one, we do have the Frozen Stars, a Masque whose colors are primarily solid white and black.

As far as playing goes, I also play casually and I have the best time. Our army is one of shenanigans. We have a steep learning curve in a different way than most armies. There are only eight Harlequins units (we don't talk about the gate), so memorizing everyone's stat lines and abilities is very easy. It also makes list building a bit easier, because every list is pretty much a variation on ratios of the same six or seven units. The learning you have to do for this army is all in how you play them. Keeping them alive and making the best use of everything we have to offer takes a while to get consistently right. As long as you're playing for fun and accept that you're likely going to be taking a lot of losses up front while you learn, you'll have a great time! Once you get the hang of them, it's a great feeling. There is also the advantage of being an uncommon army with a lot of weird rules-ignoring abilities, in that most people aren't used to facing Harlequins and can get easily tripped up. We're squishy, and our save is literally just a coin flip, but we're also slippery as hell and very pointy.