I tried this sample from the garden to look at the top and bottom, see Command/setuplayout.
Below a copy of the garden page (pagesize changed from A10 to A6):
\setuppapersize[A6]

%% This is an example for showing the effect of the vertical parameters
%% top, topdistance, bottom, bottomdistance and topspace.
%% Just play around with these parameters.

\showframe

%% Show the top and the bottom with blue color:
\setupbackgrounds[header, footer][background=color, backgroundcolor=blue]

\def\MyLength{10pt}

\setuplayout[
  top=\MyLength, 
  topdistance=\MyLength,
  bottom=\MyLength, 
  bottomdistance=\MyLength,
  topspace=\dimexpr(\topheight + \topdistance),
  height=fit]

\starttext
    \input thuan
\stoptext

This is LuaTeX, Version beta-0.75.0-2013030308 (rev 4589)
ConTeXt  ver: 2013.03.04 18:28 MKIV  fmt: 2013.3.4  int: english/english

The top and bottom should have been blue as in the screenshot from the garden (figure to the left). The reality however is no blue sections at all (figure at right). What is wrong: the (fairly recent) context or the example in the garden?

Hans van der Meer