In iOS when a button can be actioned it is coloured accordingly so the user knows to tap. This is a standard across the platform. Is there a standard for long press gestures? In the case of swiftUI the 3D Touch/ long press can show a contextMenu. How best to indicate to a user to long press? Is there a standard colour for example to show when an element is to be long pressed?
Don't think there is any such standard.
Which means you have to define what would be most relevant. Bold ? Underline ? Using color should be avoided (think of color blinded).
Seen the docs on 'handling long press gestures'?
"Overview
Long-press (also known as press-and-hold) gestures detect one or more fingers (or a stylus) touching the screen for an extended period of time. You configure the minimum duration required to recognize the press and the number of times the fingers must be touching the screen. (The gesture recognizer is triggered only by the duration of the touches and not by the force associated with them.) You might use a long-press gesture to initiate an action on the object being pressed. For example, you might use it to display a context-sensitive menu."
Remember, SUI, while still relatively new, -is- uikit. I'd just use the color change, keeping the HIG's opinions in mind: https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/color/
If there is a 'standard', it's via the HIGs, I think, which stress consistancy overall, and with your UI's current theme, meaning tint, perhaps, not strictly a mothership a set color.