Ninety-five lambs were fed as follows: lambs and dams grazing alfalfa (Gr); As Gr but lambs had access to concentrate (Gr+S); ewes grazed and lambs received milk and concentrate until weaning and thereafter concentrate and straw (Rat-Gr); ewes and lambs were stall-fed (Ind). Lambs were slaughtered at 22-24 kg of live-weight and fat and M. rectus abdominis colour measured. Visual appraisal scores of Gr and Ind were significantly different. The absolute value of the integral of the translated spectrum (SUM) was greater in Gr and GR+S. A discriminate analysis was able to discriminate between grass-fed and indoor-fed lambs. A logistic regression including SUM and b(∗) classify correctly 99.1% of carcasses. A equation is proposed to calculate the probability of one carcass to do not belongs to Gr or Gr+S group (PNA): [Formula: see text] .