It will depend on the waste being treated.  For traditional wastewater, the sludge will be darker than an AS tank

i am agree withTeresa and the sludge color is depend on SRT, but it can be dark during long srt 

I have never seen or heard about a mixed liquor color, which is different than shades of brown unless the wastewater is colored itself. The  "health" of the microorganisms are best assessed by microscopic and respirometric analyses

Dark brown is what I like to see.  Black may indicate the presence of sulfides.  I like to pay attention to the smell also.  Any of these assessments are difficult to quantify but very useful nonetheless.  A rotten egg smell is bad, this is indicative of sulfide production.  A smell similar to that of dirty laundry or a musty small is best, indicating a healthy microbial community.  Sometimes there is almost no smell... Also a good thing.  I do not like to see colorful patches such as orange or purple.  This indicates the presence of some molds or fungi and usually in my experience has been the result of poorly mixed water.  

Yes dark brown is good indicator but is see a long SRT system that it is operated in anoxic cond.  for deneitrification, its sludge and biofilm are dark.

I agree with Andrew that black colour indicates the presence of sulfides. However, anaerobic biomass is retained normally for a long time in the reactor and in that time there will be accumulation of sulfides (even if the wastewater contains a small concentration of sulfate) in the slidge. Thus, in a mature system you should expect dark brown to blackish brown colour. Yes, pungent smell of H2S should tell you more about the health than the colour.

Thank you very much. All the answers are very helpful to me.