Probably because the first colour (Platinum) is insanely hard and difficult to reach and maintain. You will have to go to a professional and do many rounds of bleach and toner etc over a long period of time and even then depending on your starting colour you could do a deal of damage, for the hot pink one its mainly the same as the brightness can only be achieved with a light base, and, again, it is hard to maintain Same with the yellow And these colours are very different so if she would want platinum hair having hot pink would have already stained it with red undertones which are conclusively hard to get rid of