Photometric measurements of 33 transneptunian objects (TNOs) and Centaurs from the target list of the Herschel Key program `TNOs are cool' are presented. Broadband filter observations of 5 Plutinos, 14 classical disk objects (CDOs), 5 scattered disk objects (SDOs), 5 detached disk objects (DDOs) and 4 Centaurs are used to determine absolute magnitudes, broadband colours and spectral gradients in the visible wavelength range. The diameters of the objects estimated with assumed average albedo values fall in the typical range for the various dynamical populations. Deviations between our and published measurements of the photometric brightnesses for three objects indicate larger lightcurve amplitudes (0.4-0.8 mag) due to non-spherical shape and/or albedo. A statistical analysis of photometric population properties using our data and those of the MBOSS2 database by Hainaut et al. (A&A 546:A115, 2012) supports the results and conclusion of this group of authors, namely it shows that dynamically cold CDOs are disjunct for their visible colours from the other TNO populations and Centaurs. Six objects (2002 GV31, 2003 AZ84, 2003 MW12, 2003 OP32, 2003 UZ117, 2005 RM43) with neutral to bluish spectral gradients were found, of which 2002 GV31 shows the smallest spectral slope among the dynamically cold CDOs known so far. Three very red objects (2002 KY14, 2004 GV9, 2007 OR10) with spectral gradients above 40 %/100 nm were found of which 2007 OR10 is by far the reddest DDO measured so far.