Was dancing with my voodoo drum for a few minutes after upgrading to xcode 8.3 today. One of my storyboards that have existed for last 3 years suddenly started to cause the compile error:
Was compiling fine just an hour ago on xcode 8.2.1! Ok, it tells something on not liking the UIColor, and log it mentions further says colors should be RGB or White space. I went by "binary search" removed everything from the storyboard except the table view -> build still fails. Only value catching the eye was "Tracking" color for the section view saying showing a black stripe with no color name, I have many of these assigned in other places though, guess it is still somehow different in this storyboard, so I reselected a "black" color for it:
2017-03-30 17:48:00.332 Interface Builder Cocoa Touch Tool[29650:5173036] *** Assertion failure in -[UICGColor encodeWithCoder:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit_Sim/UIKit-3600.7.47/UIColor.m:1549
2017-03-30 17:48:08.466 ibtoold[29647:5172916] [MT] IBPlatformTool: Wrote failed marshalling request diagnostics to path: /var/folders/8j/x30frbx948l8djrb6ff7mljc0000gn/T/IB-agent-diagnostics_2017-03-30_17-48-08_415000
/* com.apple.ibtool.errors */
/Users/stanislavdvoychenko/Documents/code/speedo/bLocNote/taxi/EditTariffStoryboard.storyboard: error: Internal error. Please file a bug at bugreport.apple.com and attach "/var/folders/8j/x30frbx948l8djrb6ff7mljc0000gn/T/IB-agent-diagnostics_2017-03-30_17-48-08_415000".
And it compiled! Huraaay and back to programming!