|Image from Flickr.com under a Creative Commons license|
This is Bart, an Australian cattle dog puppy. There is often a lot of confusion between dogs such as this one and dogs that are merle. The two colors are actually quite different, being controlled by different sets of genes.
To begin, Bart has tan points. This is clearly seen despite the fact that the majority of the dog is pale. Since recessive black is not seen in the breed, it's safe to say that Bart must be atat tan pointed.
Next, the red seen on this dog is moderate. As such, he's most likely cchcch moderate red.
Bart is also roan. Roan dogs have white markings that have become filled with evenly distributed colored hairs. Roan appears to be nearly fixed in the breed, and as such Bart is most likely RR roan.
Next, for the roan on Bart to be visible, he must be expressing one of the genes on the Spotting locus. Judging by the size of the dark patches seen on him, he is most likely swsw extreme white piebald.
Lastly, it's believed that roan is a modifier that acts on the ticking gene. As such, for a dog to be roan is must also be ticked. Since Australian cattle dogs appear to be nearly fixed for roan, they must also be nearly fixed for ticked. As such, Bart is most likely TT ticked.
So, that would be atat cchcch RR swsw TT or extreme white piebald black and tan roan (with moderate red).
As I already mentioned, merle and roan are often confused, especially in breeds such as the ACD. Since so many herding breeds are in fact merle and the two colors can appear quite similar, it's easy to see why people could become confused. There are, however, some easy ways to tell the colors apart. As newborns, merle dogs will look very much like they will as adults. However, dogs such as Bart are born with white that colors as they grow. All dark patches seen on the adult will be present at birth. The tan markings can also be telling, since merle usually does not affect red pigment. Any tan points on a merle are most often quite clearly visible with no splotchyness or paleness, while on a roan dog the roaned red is just as pale as the roaned black. Another simple way to tell the difference is to look for white. If a dog has clear white markings as an adult, it is most likely merle. If there is no clear white, then it is quite possible that the dog is roan. Dark patches on merles are also more random than patches on roans, as the patches on a roan will appear in the same fairly predictable orientations as seen in any dog with the Spotting locus in action. Roan is also far more likely to breed true if two roans are bred together, partly because the roan gene is not linked to any health issues. In contrast, breeding two merles together always results in only 50% merle along with 25% non-merle and 25% double merle, the latter having a very high incidence of health issues. All of these things combined can make identifying whether a dog is roan or merle quite easy.
To make things more confusing, you can have dogs that are roan and merle!