aircraft down to the last kg
The LDS usually will quote mass to the kilo and CG to the millimetre. However, the reality is that the numbers are comparatively approximate due to the limitations of weighing equipment and errors in replicating the calibration conditions at the weigh.
The 10% colour rule may be aircraft specific to some composite aircraft and is an manufacturers requirement.
Indeed, that may be the case and would apply in principle to any composite structure. One would be a bit adventurous painting a composite totally black without some data on potential structural consequences.
However, that is quite different to weight and balance considerations.
And then 250-350 pax walk on all weighing 5-10kg more than 'standard'
True but a matter for operator protocol and procedural discipline ..
as we get more obese my the minute
A basic premise underlying the use of standard weights (but generally ignored for obvious reasons) is that the operator uses an appropriate population statistic where appropriate means that the standard weight relates to the real population in question.