I second that opinion that colors should be used to indicate the level of constraint...
However I also would like to suggest to #F__k constraints! Why be so restrictive all the time?
I would suggest to implement an option to not care for constraints! Good enough if the sketch is stable in a given moment!
If somebody chose to work with something that is not fully constraint, maybe his design will go all bezerk some day, but who cares it happens anyways for so many other reasons...
Also the inability to recognize, highlight, and offer to reference any edges, planes origin points appears arbitrary, and should never been disabled inside a sketch (or in the worst case as a non default option! Default should be to be able to select any geometry that can be easily deducted to the normal of the plane that is being worked on, and in case somebody initiates a reference to such geometry give them a dialog that says this will require to reference 'external' geometry 'are you sure you want to proceed'.
If there is any ambiguity on which plane, object, body etc. a feature should be applied to this relation should be an option that should be modifiable. I know this requires to have a full parametric design tree in which the sequence of features makes a difference. The capacity to roll back and reorder features would require some discipline from the designer, but it will enable things that even very expensive design packages may not fully support!
For example I am struggling because I haven't even figured out how to reference to such a universal concept such as the origin point (or 'XZZ zero planes'), which should be possible by default on any sketch plane.
I am proposing to be warning but not restricting - this is the essence of enabling creativity!
Restriction require code too!
I would like a streamlined interface where where mouse over highlights anything without restrictions,
Right click asks the highlighted or selected object what methods it supports and which data it can deliver
If for example a highlighted surface is flat it could deliver a mathematical plane this should be offered to intersect with any other plane (if you are in a sketch offer to add a construction line based on this intersection!
If it's always straightforward enabled by default, disable if you must - you will be surprised how fast we get to a super powerful and quick to learn interface!