Too much water. Nothing needs constant water, especially in December, even in So Cal. Let the soil dry on top before watering again. If it's in a pot make sure the water drains out the bottom. Potted roses in So Cal might need daily water most of the year, but not in December, unless they're extremely rootbound. And roses in the ground do not need daily water unless they're in very sandy soil. As I recall, most of So Cal has clay, and clay stays wet for a LONG time. Clay soil presents another problem - getting the water in deep enough. Takes way longer than you would think. When I gardened in NorCal on heavy adobe clay, I would use drip irrigation to water the roses - five drippers at a gallon an hour, for about an hour, two or three times a week in the hottest weather, and not at all in winter. Minis would need only two or three drippers. when you say full sun, that's for at least 6 hours, right? Sometimes when people say 'full sun', they only mean that it gets direct sunlight at some point of the day, even if just a few minutes. Cold??? what do you mean by cold?? My roses are still nice dark healthy green, and they've been snowed on twice this month. Our temps have typically been in the upper 30s - low 40s, with occasional dips to upper 20s. Has it been colder than that where you are? They've also gotten nearly a foot of rain this month, but they're all in very well drained soil. We typically get about 55 inches of rain a year, but we also typically have gravelly and sandy soils, not clay.

I had precisely the same situation: a store-bought plant that was doing great for a while under fluorescent light (a CFL in a gooseneck lamp), then the lower leaves started yellowing, showing spots, etc. I took a really close look under a low-power microscope, and found out I had mites. These little suckers are nearly transparent, unlike the black ones that are relatively easy to spot. I believe that the plant cuts off nutrients to the infested leaves as a defense mechanism, since the mites can't suck the juices from a dry leaf. Use a miticide if you're OK with chemical treatments; Bayer's 3-in-1 has a particularly effective agent in it. If not, or if your state hasn't approved the stuff for sale (you cannot buy it in NY), you can try my approach: fill a bucket with soapy water (dish detergent, ca. 1/2 tsp per gallon), and shove the entire plant, upside-down, into the water. Slosh it around, and up-and-down, for about a minute. This removes the bugs, drowns the ones who won't let go, and I think it might even damage the eggs (tiny white specks on the bottom of the leaves.) Repeat once a week for at least 3 weeks (the mites have a 10-day life cycle), and maybe once a month after that to keep 'em down. The mites prefer a dry environment, so this treatment, along with misting, will prevent them from doing noticeable damage. I think the real problem with keeping these plants indoors is that the mites are not exposed to their natural predators, so their population can explode very quickly. They are incredibly persistent, and they will keep coming back time and time again if you don't manage to kill off every last one of them (and every last egg as well!) Unless you like having ladybugs crawling around the house, you're stuck with regular washing, or chemical treatments, to keep them under control. Here is a link that might be useful: Article on Spider Mites

What did you use for the Aphids? Leaves turn yellow, exhibit chlorosis, for many reasons and sometimes people spray inappropriate things on plants to eliminate Aphids that cause the plants leaves to die, which yours are doing. If something was sprayed on those leaves that restricted that leafs ability to breath or photosynthesize the plant will cut off nutrient flow to that leaf and it will die.

I have a stack of mini roses as a border. I find that as soon as I put them new in from the nursery they go into transplant shock a little and crisp up no matter if you do water and feed them. I think its just a matter of the roots adjusting a little. They've usually been in a state of major flowering when you get them from the nursery.They fine up eventually for me, with a little care (I've killed a couple, now I do the seasol and major watering thingy). It's probably what you get for planting in Bris summer heat. They seem to struggle with the dryness in well fertilised soil a little as well and they get chomped a bit. I don't recommend rose spray at that stage. Now I just go for the biggest miniature I can find as they hold up a little better under pressure. It might just be my lack of skill, but to get them back to the condition they were in and flowering takes a few weeks. Sunburn sounds right. I also debud them because I think it gives them a better chance. I don't lose them now LOL. Happily the icebergs are thriving and my knee high plants have around 30 roses weighing them down. They looked like a dried floral arrangement up to a month after i planted them (in potting mix topsoil.....yes I figured out that was not a good idea). The flower carpet roses seem to be duds but maybe they need more time. And I haven't killed my latest rose aquirements = yes there's a Kordes and a David Austin there. They're not doing brilliantly - but they are alive *S*. They're not crisping up in the March rain. Yeah! I gather that natural fertilisers may give new plants a better start.....