When the length is adjusted do the adjustments stay put? I have a peak design where I need to keep shortening the strap.

I have two Leash straps and after adjustment they stay put. I have a Peak Design Slide Lite and two Leash straps. None of these straps has slid out of adjustment, even with heavy camera and lens combinations. In fact, I can forcefully pull on the ends of the straps and they do not budge. If you continue having trouble I suggest you contact Peak Design directly as I'm sure they'd be very interested to know if one of their products were failing!

Does this strap have a smooth and grippy side like the slidelite strap?

Unfortunately, no. I do not use a sling or a strap around the neck, as I find both very uncomfortable. Over forty years having a camera on my shoulder and it's a hard habit to break. I purchased two of these and they slide right off my shoulder. they're built very well from excellent material, but not made for those who use large DSLRs from the shoulder. I have purchased a "shoulder replacement" with a grip side. I hope it works. that's an option if you want to purchase this strap, otherwise, it's a sling or around the neck.

Available in 2 colors, ash or black, but I only see black?

Colors are stocked individually, if you are only able to see one variation within the Amazon Listing, it means that the other variation is currently out of stock. We have more stock constantly flowing into inventory. Best wishes, Adam

would this strap work with a dslr or is it best for just mirrorless?

The strap is very steady and great build so I think it works for a DSLR as well but I would recommend you consider to buy a Peak Design Slide Strap SL-1 for a medium size DSLR such as D7500 .... or canon 7D... or SL-2 for a bigger camera like full frame. I use this strap for my sony A7R and It looks very nice and very comfortable. The best strap I have ever used so far!