Color management is definitely complex and confusing.
Sketch’s hand was forced to add color management (I believe it’s in the latest beta?) by the crop of new displays (new Macbook Pros, iMacs, iPhones among others) that support color spaces with wide gamuts like P3. They were previously relying on that fact that most displays use the sRGB color space, so they weren’t doing any conversion.
Even with the crop of new displays, sRGB remains the lingua franca of digital work. For the near future, most displays are still going to be sRGB. If you design against a wider gamut, you’re risking that the color you specified won’t render correctly for most users.
Working in sRGB means that colors will render similarly across different browsers and computers. Most browsers convert from sRGB to the display’s profile—and this is also what Subform does. So if your target is sRGB, you’re good.
One thing to note: if you’re working in Subform on a wide-gamut display, sampling colors off the screen with an eyedropper tool isn’t going to be a reliable. There’s a boring long technical reason for this, but the short of it is that the values reported by the picker can be slightly off from the actual specified/rendered value.
An exception would be if you are designing for only wide-gamut displays on a wide-gamut display—say, you’re designing an app that only runs on iPhone 7+/X on your iMac. In this case, you might want to take advantage of the extra color options and avoid the sRGB color space altogether. This isn’t something we support in Subform right now.
Longer term, as displays that support DCI-P3 and Rec.2020 become more prevalent, we’re unfortunately all going to have to move from not thinking about it to being on top of color spaces and conversion. Referencing color profiles is coming down the pipe in CSS at some point (check out this article from Apple and keep an eye on the CSS working group’s progress with the Color Module Level 4.)
We’ll see how things shake out. Hopefully, it’ll still be much less complex than color management for print work. 
