Context :

Aims : With the aim of increasing the sample of M31 clusters for which a colour-magnitude diagram is available , we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M31 globular clusters RBC Version 3.5 available at : www.bo.astro.it/M31 .

Methods : Sixty-three such objects were found .
We used the ACS images to confirm or revise their classification and were able to obtain useful CMDs for 11 old globular clusters and 6 luminous young clusters .
We obtained simultaneous estimates of the distance , reddening , and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way .
We estimated the age of the young clusters by fitting with theoretical isochrones .

Results : For the old clusters , we found metallicities in the range -0.4 \leq [ Fe/H ] \leq - 1.9 .
The individual estimates generally agree with existing spectroscopic estimates .
At least four of them display a clear blue horizontal branch , indicating ages \ga 10 Gyr .
All six candidate young clusters are found to have ages < 1 Gyr .
The photometry of the clusters is made publicly available through a dedicated web page .

Conclusions : With the present work the total number of M31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters , and from 7 to 11 for the young ones .
The old clusters show similar characteristics to those of the MW .
We discuss the case of the cluster B407 , with a metallicity [ Fe/H ] \simeq - 0.6 and located at a large projected distance from the centre of M31 ( R _ { p } = 19.8 kpc ) and from the major axis of the galaxy ( Y = 11.3 kpc ) .
Metal-rich globulars at large galactocentric distances are rare both in M31 and in the Milky Way .
B407 , in addition , has a velocity in stark contrast with the rotation pattern shared by the bulk of M31 clusters of similar metallicity .
This , along with other empirical evidence , supports the hypothesis that the cluster ( together with B403 ) is physically associated with a substructure in the halo of M31 that has been interpreted as the relic of a merging event .