5. If the scales showed the exact weight, the sum of the results of the first three weighings would give twice the total weight of the three portfolios. However, the scales are inaccurate, and the total error over the three weighings is no more than 1.5 kg. Therefore, the three portfolios together weigh no less than $\frac{5.5+7+6-1.5}{2}=8.5$ kg. On the other hand, the fourth weighing shows that the three portfolios together weigh no more than $8+0.5=8.5$ kg. Therefore, the portfolios together weigh exactly 8.5 kg. This is possible only if the weight was maximally overestimated in all three of the first weighings, i.e., by 0.5 kg, otherwise the three portfolios together would weigh more than 8.5 kg. Therefore, the true total weight of Petya and Vasya's portfolios is 5 kg, Petya and Kolya's portfolios - 6.5 kg, Kolya and Vasya's portfolios - 5.5 kg, from which the answer is easily found.
Answer: Colin - 3.5 kg, Petya - 3 kg, Vasya - 2 kg.