I have heard you shouldn’t paint timber substrates in dark colours. Is this true?

It all comes down to the Light Reflective Value (LRV) of the paint you are considering using.

The approximate light reflectance of a colour indicates the amount of visible light that a colour will reflect. Black has a LRV of 0% and absorbs all light. The surfaces are consequently very dark and can get very hot. In contrast, white has a LRV of 100% and keeps substrates light and cool. All colours fit within these two extremes.

Mid to dark colours are not suitable for some exterior substrates and if used may cause damage to the substrate, such as warping, checking and premature failure.

Resene CoolColour™ in some instances can be an option and for best results it is suggested that a white undercoat be used.

Always check with the substrate supplier prior to commencing any work to ensure the colour selections fit within their guidelines and these guidelines should be followed particularly if any substrate guarantees are involved.

The following links may be useful when making a decision:

Light Reflectance Values (LRV) Colours and Heat Generation
Resene CoolColour™ Technology
Resene CoolColour™ Brochure
Coatings for Claddings
Information for Architects on LRV