As an Austrian native speaker, I'd opt for the Umlaut in most of these examples. Most definitely for 'schmäler'.

That said, I wouldn't be surprised if other native speakers choose differently, or express uncertainty in how to form the comparative correctly. Many of these words are rarely used, and there are often alternatives.

As an Austrian native speaker, I'd opt for the Umlaut in most of these examples. Most definitely for 'schmäler'.

That said, I wouldn't be surprised if other native speakers choose differently, or express uncertainty in how to form the comparative correctly. 

Many of these comparatives are rarely used in day to day conversation, and there are often alternatives. There are specific names for many colors, and being wet is often considered an absolute state. Still, the comparative exists, if only to be able to say

Nässer kann ich ohnehin nicht mehr werden.

Something that is not completly wet is either only 'feucht' (moist/damp), or only some parts are wet, etc.