It’s common to see wooden pallets with streaks or patches of colour—green, blue, grey, even black. At first glance, this may raise concerns about mould or contamination, mainly when pallets are used for food, pharmaceuticals, or consumer goods. However, not all discolouration is harmful. Many colour changes in the wood occur naturally and do not affect safety or performance.
Here’s how to understand the most common colour variations found on wooden pallets and what they typically indicate.
Understanding Natural Wood Discolouration
Not every unusual shade is a sign of mould. In many cases, colour changes result from natural reactions in the wood, minerals in the soil, or contact with metals.
Green or Purple Streaks
These often come from mineral deposits within the tree as it grows. Wood sourced from mineral-rich soil may develop natural streaks—green is common in certain maple varieties, while purple can appear in species such as poplar.
Impact: Harmless, no effect on the pallet’s structural strength.
Blue Stain
This may look like mould at first, but it is usually bluestain, a type of wood fungus that causes pigmentation but does not break down the wood fibres. It is not airborne and does not compromise strength.
Impact: Aesthetic only; wood remains structurally sound.
Black Marks
Known as iron stain, this occurs when metal fasteners, nails, saw blades, or steel tools come into contact with wet or tannin-rich wood. The chemical reaction creates dark marks.
Impact: Harmless, but may look unattractive.
Grey Tones
A natural enzymatic reaction in hardwoods such as oak, beech, or maple can cause sapwood to turn slightly grey. This often happens while the wood is still drying.
Impact: Natural variation, no safety concern.
When Colour Can Signal a Problem: Mould Indicators
Although many colour changes are harmless, mould is still a possibility—and it can present in multiple colours, including blue, green, yellow, grey, black, and white. To determine whether a pallet has mould, look for:
- Soft, fuzzy, slimy, or damp surface texture
- A musty or rotten odour
- Visible growth that spreads over time
If these signs are present, the pallet should be isolated, cleaned, or removed from circulation, especially in hygiene-sensitive industries.
Key Takeaway
Not every coloured streak or spot on a pallet is cause for alarm. Many colour variations are natural, harmless, and purely cosmetic, with no impact on performance or safety. Understanding the difference between benign discolouration and mould helps ensure safe use while avoiding unnecessary disposal of perfect pallets.
Recognising these variations supports more brilliant pallet handling, reduces waste, and maintains quality across your supply chain.