Class XI Math Chapter-7 Example 15 NCERT Book

Sir I am in doubt that if in required  arrangements,discs of same colour to be indistinguishable, if and only if when same colour's discs are to be placed together and to be treated as single entity and then make such arrangements ie, 3*2*1=6 total six arrangements where discs of same colour be in indistinguishable. 

As per provided soln in book also, we can distinguish each colour disc to another one if placed alternatively. I think you can understand my doubt. Pls make me clear by giving one each example of arrangements,where it's distinguishable or when it is indistinguishable. Thanks.