My wife is a Black and White film photographer. I was wondering if there is a color code that I could use as a basis for a helper app?
Typical phone screens have too broad a spectrum to use as safelights (there's too much orange in the red channel). The red safelight spectrum here indicates that there's no light below 600nm. You'll get too much leakage below that from any LCD. With OLED you might be OK I've seen spectra that indicate that you would, and that you wouldn't (e.g. here, though there's no data on how they took the spectra). So using a phone as a safelight seems to be out. The main effect would be a reduction in contrast.
If you want something like a timer app, you might have more luck. OLED might be good here as the black background would be really black rather than leaking short wave light.
For a red-on-black timer app, unless you point it directly at your paper, you're likely to get away with it even with LCD as the photon count is so low, especially if you keep the backlight turned down. The red would be #FF0000.
Of course, if you're using your phone in the darkroom and a bright white notification pops up for a text, your print is probably ruined wherever the phone is pointing.
If you had sheets of Rubylith masking film by ULANO™ you could lay a sheet over any device for a darkroom safe-light filter. If you peel off the strip-able layer, and it will cling right to the phone or pad screen as a Mylar layer does without any glue or moisture. It is perfectly transparent and you can see the tiniest details through the red layer. It's also waterproof like plastic wrap so it's darkroom friendly. I have a lifetime supply here in Montreal. :)
Edit: If you use Rubylith over a fluorescent tube, it must be replaced every few weeks as the UV bleaches the red dye used in the film rendering it useless.
Try this with iPhones running iOS 10.1 and higher: Go to Settings > General > Accessibility > Display Accommodations > Color Filters > slide to 'on' (right) position Then, in the drop-down menu that appears, select 'Color Tint' and slide INTENSITY and HUE to the extreme right side.
This will shift the display to a photo-safe red, and should be fine for viewing any app in situations requiring red safelight work. That said, I would still recommend reducing the overall screen brightness level to less than 50%. If you slide the upper screen display's palette viewer from the default pencil array to the middle ROYGBP grid, you will notice that the 'actinic' green to purple wavelengths are now displaying as variations of mid red/grey to black.
To switch back to the normal display, go to Settings > General > Accessibility > Display Accommodations > Color Filters > slide to 'off' (left) position.
From this point on, every time you want to switch to the red channel display, navigating to Color Filters and sliding to the right will turn the display to red.
I teach photography and have worked in wet lab situations for over 30 years, and this little-known gift from Apple has saved a lot of photo materials from unwanted exposure!