Hi Antonin
I would use BEDTabix because the BED handling of flatfile-to-json doesn't load the itemRgb info correctly as far as I can tell, but with BEDTabix, it's fairly easy.
Example config for a BEDTabix track
{
"label": "My track",
"urlTemplate": "file.bed.gz",
"storeClass": "JBrowse/Store/SeqFeature/BEDTabix",
"style": { "color": "function(feature){return feature.get('itemrgb')}" }
}
-Colin