qgis - PyQGIS save raster as rendered image, then use GDAL tools on it

I have tried to use the code from QGis Save Raster as Rendered Image. i is a raster layer:

pipelayer = i
pipeextent = pipelayer.extent()
pipewidth, pipeheight = (pipelayer.width(),
                         pipelayer.height())
piperenderer = pipelayer.renderer()

pipeprovider = pipelayer.dataProvider()
crs = pipelayer.crs().toWkt()
pipe = QgsRasterPipe()
pipe.set(pipeprovider.clone())
pipe.set(piperenderer.clone())
pipedFile = os.path.join(tempfile.gettempdir(),
                         safeLayerName + '_pipe.tif')
print pipedFile
file_writer = QgsRasterFileWriter(pipedFile)
file_writer.writeRaster(pipe,

                        pipewidth,
                        pipeheight,
                        pipeextent,
                        pipelayer.crs())

in_raster = pipedFile
prov_raster = os.path.join(tempfile.gettempdir(),
                           'json_' + safeLayerName +
                           '_prov.tif')
out_raster = dataPath + '.png'

crsSrc = i.crs()
crsDest = QgsCoordinateReferenceSystem(4326)
xform = QgsCoordinateTransform(crsSrc, crsDest)
extentRep = xform.transform(i.extent())
extentRepNew = ','.join([unicode(extentRep.xMinimum()),
                         unicode(extentRep.xMaximum()),
                         unicode(extentRep.yMinimum()),
                         unicode(extentRep.yMaximum())])
processing.runalg("gdalogr:warpreproject", in_raster,
                  i.crs().authid(), "EPSG:4326", "", 0, 1,

                  5, 2, 75, 6, 1, False, 0, False, "",
                  prov_raster)
processing.runalg("gdalogr:translate", prov_raster, 100,
                  True, "", 0, "", extentRepNew, False, 0,
                  0, 75, 6, 1, False, 0, False, "",
                  out_raster)

It doesn't work, instead giving me the unstyled raster as final output. I really don't understand the problem, because the output of writeRaster (the file pipedFile) is styled, so the process from the answer linked to above is working. It's just that when I try to use the output image to run through GDAL's warpreproject and translate, it somehow reverts to the unstyled raster.

What have I done wrong?

UPDATE: The output of QgsRasterFileWriter is styled. The output of warpreproject is not styled, but the .tif is accompanied by a .aux.xml file which appears to have colour information in it:

  
    
      
        -0.498046875
        255.498046875
        256
        0
        1
        314|4|3|2|3|5|4|4|5|8|10|9|16|16|23|26|37|46|58|62|69|77|77|94|127|94|131|136|133|162|169|172|171|184|200|186|207|196|186|177|179|194|182|182|178|177|181|195|196|196|195|195|179|226|206|215|193|197|202|212|207|206|232|204|229|253|233|240|248|234|234|268|240|238|259|286|268|287|259|264|255|267|268|271|256|321|277|284|317|286|290|277|312|334|319|325|323|310|349|357|347|326|337|338|336|383|374|349|411|382|382|417|406|405|414|427|404|434|447|430|468|424|446|442|450|461|448|458|457|459|412|414|469|466|443|475|464|472|481|519|504|458|473|481|514|523|522|494|542|580|604|623|686|648|707|763|808|799|819|853|932|1046|1062|1118|1145|1215|1293|1393|1335|1392|1390|1317|1232|1247|1215|1088|1032|896|847|731|708|666|608|634|556|500|484|456|433|408|406|401|386|388|414|414|387|382|340|346|382|385|368|348|334|376|359|318|339|326|357|315|319|357|333|328|329|310|344|295|300|351|347|324|358|349|363|361|347|396|386|433|395|434|479|473|523|550|644|655|672|778|882|1014|1183|1400|1681|2075|2531|3529|4851|7235|10793|60773|7297|1999

      
    
    
      255
      203.31482680479
      0
      63.158702013579
    
  

The output of translate is not styled, and has no additional file. However, since it's a PNG not a GeoTIFF, that's perhaps the explanation.

I've tried looking at the expand option of translate, but that gives me image files which do not display in a web browser - I've not investigated why.

UPDATE 2: No, using expand results in no output file at all. Wondering about trying pct2rgb.

UPDATE 3: I'm wondering if file locking is causing the problems. Could it be?