Colours (including single colours) used in relation to goods/services are registrable as trade marks and there are many colour marks on the Register. Section 6 of the Act provides the definition of a sign:
sign includes the following or any combination of the following, namely, any letter, word, name, signature, numeral, device, brand, heading, label, ticket, aspect of packaging, shape, colour, sound or scent.
The assessment of a colour trade mark under section 41 is no different from the assessment of any other trade mark: Kenman Kandy Australia Pty Ltd v Registrar of Trade Marks (2002) 122 FCR 494 at  per Stone J.
In BP p.l.c. v Woolworths Limited (2004) 212 ALR 79, Finkelstein J found (at first instance) that three applications for “the colour green” (being trade mark nos. 559837, 559838 and 559839) were not inherently distinctive and further commented that single colour trade marks could never be inherently distinctive, but could only be protected by evidence of acquired distinctiveness. He then stated (at -), specifically in relation to the colour green, that:
Whichever test is applied the colour green (in the particular shade shown on the applications) is not inherently distinctive of BP’s goods and services. Applying the negative test, green is not inherently distinctive because, as I will soon show, it is a colour used quite innocently by other owners and operators of service stations. The colour is not inherently distinctive according to the positive test because the colour is simply descriptive. The marks will only be distinctive of them if, by the filing date of each application, the colour had acquired a secondary meaning.
Most objects have to be some colour. So merely applying a colour to a product will not act as an identifier for that product. In deciding whether colour functions as a trade mark it is necessary to determine whether the trader has used the colour in a way that informs the public that the product emanates from a particular source. Put another way, colour must be used to distinguish products and not as mere ornamentation or decoration.
The Full Court overturned Finkelstein J’s decision in relation to whether the Applicant’s evidence was sufficient to distinguish the Applicant’s goods and services under section 41(6) but it was accepted on the appeal that the mark had no inherent capacity to distinguish. The only issue (and the issue on which the appellant was successful) was whether its use of the mark was sufficient to prove distinctiveness in fact as at the priority date.
Some recent decision in which colours (per se) or words for colours have been rejected for being incapable of distinguishing the applicant’s goods or services include:
YELLOW in respect of a broad range of goods and services including print and online telephone directories: Phone Directories Company Australia Pty Ltd v Telstra Corporation Limited (2014) 106 IPR 281; upheld on appeal Telstra Corporation Limited v Phone Directories Company  FCAFC 156.
RED in respect of printed matter, newspapers, magazines etc: Major League Baseball Properties Inc v The National Magazine Company Ltd  ATMO 35.
in respect of pharmacy retail and wholesale services: Discount Drug Stores Pty Ltd  ATMO 66. (This mark was considered to be somewhat, but not sufficiently, inherently adapted to distinguish. Insufficient evidence of use.)
This decision of the delegate was set aside by the Court following agreement between the parties: Discount Drug Stores Pty Ltd v Registrar of Trade Marks  FCA 1159.
Similarly, in Grove Fruit Juices Pty Ltd  ATMO 1, an application for a trade mark consisting of “the WHITE COLOUR of the bottle in which the goods are sold” was rejected as being incapable of distinguishing.
And, a trade mark consisting of green striplight (neon, LED or similar) affixed to the edge of the canopy over the petrol pumps at vehicle service stations (in respect of fuels, retail of fuels, vehicle refuelling etc, take away food services etc) as shown below was rejected in Woolworths Limited v BP p.l.c.  ATMO 61.