every time the MS reselects a cell which has a new Routing area code or routing area color, then it will do a routing area update.
impact : signalling load on the SGSN, TBF / PDCH usage on the air interface, etc. it is highly recommended to reduce the number of RA update !
typical solution :
within one LAC, use only one RAC : all cells in this LAC shouldshare the same ra code and ra color.
in next LAC, use another RACode and RAColor.