The claim that light-coloured clothing affects performance deserves a colder look

The physical starting point

Pale fabrics reflect more incident radiation than dark ones, so a garment colour does affect how much solar energy is absorbed at the surface. That is a genuine physical difference and it is the basis of the intuition that lighter kit should be cooler in direct sun. The intuition becomes unreliable once the garment is considered as part of a system that includes air movement, sweat and the body underneath.

Heat leaves a working athlete predominantly through the evaporation of sweat, and evaporation depends on fabric permeability rather than on colour. A pale garment that traps humid air against the skin can be less effective than a darker one that allows vapour to escape.

Why the comparison is hard to run

Testing the effect properly requires matched garments differing only in colour, worn in identical conditions by comparable athletes performing identical work. Matching everything else is difficult because dyeing processes can alter fabric weight and finish, so colour rarely varies in isolation. Conditions also vary continuously outdoors, and the radiant load depends on cloud cover, court orientation and the angle of the sun.

Under those constraints, any difference attributable to colour is likely to be small relative to the variation between sessions. Small effects buried in large variation are precisely the effects that require careful design to detect and that casual comparison will miss.

What the tradition argument is actually about

The historical rationale concerned appearance rather than physiology, specifically the visibility of perspiration on coloured fabric. That is a social objection with a physical basis, and it explains the rule without requiring any claim about athlete performance. Defending the requirement on thermal grounds imports a scientific justification for a decision that was made on other grounds entirely.

The imported justification then has to bear a weight it was never designed for, which is why it attracts scrutiny it need not have attracted. A convention defended as a convention is much harder to attack than one defended with an argument that can be examined.

The undergarment complication

Restrictions extend to layers beneath the visible garment, which is where much of the practical thermal and comfort function actually resides. A rule reaching that far interacts with individual needs that vary considerably between athletes for reasons unrelated to the sport. Those needs are not matters for public argument, and a rule that fails to accommodate them creates problems that colour physics does not address.

The relevant question there is administrative flexibility rather than reflectance, and conflating the two confuses both. Anyone with a specific medical or physiological concern about clothing and heat needs a qualified professional rather than a general argument.

Where that leaves the claim

Colour has a real but modest effect on radiant absorption, and fabric construction has a larger effect on the route by which most heat is lost. A claim that pale kit meaningfully improves performance in competition has not been demonstrated and would be difficult to demonstrate. A claim that it makes no difference at all overstates in the opposite direction, since the radiant component is not zero.

The accurate position is that the effect exists, is probably small and has not been isolated under match conditions. That leaves the rule to be defended on the grounds it was actually adopted for.