Written & reviewed by Marcus ReedIICRC WRT
Reviewed August 30, 2026· Next review Feb 2027
Quick answer
Mold colour comes from pigments, spores and what the mold is eating, and the same species can appear in several colours — so colour does not identify a mold and does not indicate how harmful it is. In homes, black or dark green growth is usually Cladosporium, Aspergillus or Alternaria and only sometimes Stachybotrys; white growth is often early-stage mold or mildew, or on masonry may be efflorescence (mineral salts, not mold); green is typically Penicillium or Aspergillus; orange and red on damp bathroom surfaces are usually Serratia bacteria or a yeast rather than mold; yellow can be Aspergillus or, on wood, a slime mold or fungus. CDC's position is that visible mold is removed regardless of type — the response is identical for every colour: find the moisture, remove the growth, dry the material.
Why Colour Is a Poor Guide
Mold colour comes from three things: pigments the species produces, the colour of its spores, and what it is growing on. The same species grows white when young and green or black when it sporulates; the same colour appears in a dozen unrelated species. So colour tells you a little about *what* and nothing reliable about *how harmful*. CDC and EPA guidance sidesteps the whole question: visible mold is removed regardless of type, and sampling to identify it is generally unnecessary.
What colour *can* do is give a rough idea of where you are — and rule out a few things that are not mold at all.
Colour by Colour
| Colour | What it usually is in a home | Where | Note |
|---|---|---|---|
| Black / dark green | Cladosporium, Aspergillus, Alternaria; sometimes Stachybotrys | bathrooms, window frames, wet drywall | Stachybotrys means long-soaked material — black mold symptoms separates fact from myth |
| White | early-stage mold or mildew; some species stay pale — or efflorescence on masonry (salt, not mold) | closets, basements, concrete walls, wood | chalky powder that brushes off dry from concrete is salt; white mold in the house covers the rest |
| Green | Penicillium, Aspergillus, Cladosporium | food, damp fabric, walls, wood | the classic "bread mold" family; very common indoors |
| Grey / brown | Alternaria, Ulocladium, mature Cladosporium | window sills, showers, cold walls | grey-brown velvet on a sill is the condensation mold |
| Pink / orange-red slime | Serratia marcescens — a bacterium, not mold | grout, shower curtains, drains, toilet bowl | pink shower slime: cleans off, returns unless the shower dries |
| Orange (fuzzy) | a true mold, or a slime mold on wood | damp wood, food, mulch | on structural wood, a sign of persistent wetness |
| Yellow | Aspergillus species; on wood, a "yellow slime" or fungus | food, damp wood, insulation | on framing or subfloor, treat as a wet-wood problem |
| Blue / blue-green | Penicillium | food, damp organics | the antibiotic's namesake; a food and fabric mold |
| Fuzzy vs slimy | fuzzy = mold (fungal); slimy film = bacteria or yeast | — | a useful distinction in bathrooms |
The Two Colours That Aren't Mold
Pink and orange slime in wet bathroom areas is Serratia marcescens, a bacterium living on soap residue. It cleans with a bleach-based or peroxide cleaner and a brush, and is prevented by drying the shower after use. It is an opportunistic pathogen for the vulnerable, which is reason enough to keep on top of it.
White crystalline powder on concrete, brick or block that brushes off dry is efflorescence — mineral salts carried to the surface by water evaporating out of the masonry. It is not alive and not harmful, but it is a moisture report: that wall is wet from behind, and the basement humidity and seepage rules apply. Fuzzy, spreading white growth with a musty smell is mold.
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What Colour Doesn't Tell You
- How dangerous it is. The molds that produce mycotoxins come in several colours, and most dark mold is not one of them. The exposure evidence does not sort by colour.
- Whether it's "toxic." CDC calls the term inaccurate; no home test identifies it by colour.
- How to clean it. Every colour gets the same method: fix the moisture, scrub hard surfaces and replace porous ones, dry the area, and a professional above about 10 square feet or in HVAC.
What Colour and Texture Together Do Tell You
Heavy, dark, slimy-then-powdery growth on drywall or ceiling tile says a material has been saturated for a long time — a leak, not condensation. Light speckling across a bathroom ceiling says steam and a weak fan. Grey-brown velvet on a sill says a cold window in winter. Pink slime says soap and standing water. Use the colour to guess at the *water*, then go and fix the water.
Bottom Line
Black isn't necessarily Stachybotrys, pink isn't mold, white may be salt, and no colour changes what you do. Read the colour as a hint about where the moisture is coming from, then treat every one of them the same way: find the water, remove the growth, dry the material.
Frequently Asked Questions
Is black mold the most dangerous colour?
What is orange or red mold in the bathroom?
Is white mold dangerous?
Do I need to identify the mold colour before cleaning?
Sources
- CDC — Basic Facts About Mold and Dampness
- EPA — Mold Cleanup in Your Home
- CDC — Facts about Stachybotrys chartarum
Methodology: How we source and verify data · Report an error
Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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