Blue is hard because you have to breed hybrids, that produce non-hybrids, which are coded as "special", and can then be used to make blue roses. So yes, that's hard. Purple roses? Idk, ask someone who's actually tried to breed them. I like sticking with the black roses.
I don't think purple roses are too hard to breed. They show up pretty easily for me, anyway! I got my first couple from a white rose and a white rose. Once you get purple roses, you can make the breeding easier by using a purple rose and a purple rose. I still use a few sets of white x white roses just because, though.
I believe in previous games, how you planted the flowers mattered some, but I'm not sure if that's the case in New Leaf. I use the diagonal pattern, because that's just what I'm used to, but I've had unintentional hybrids pop up when flowers were planted next to each other as well.
blue roses and purple pansies are annoyingly difficult. blue pansies and purple roses are not. but yeah, if you want any of the former i suggest buying a pair and breeding those >.<