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WiltedWisp
Everyone's already given some good tips! Sorry if this is too much information, but I have okay luck selling dragons, so here are my suggestions.
It's a good idea to think about eye color and theming. This goes double since you're in Plague, because the red eyes are kind of divisive. I have good luck selling dragons with monochrome (white/black/grey), red, brown, or even sometimes green color schemes, or some combination of those colors. Anything else, and it's very hard to sell them, no matter how nice they look, just because the eyes clash.
If you want to breed other color schemes, you can always rent or borrow a nest from another player (there should be a big nest rental network or two in Find a Dragon where people advertise) for eyes that match better. I had a pair that made purplish dragons, and the difference in sales when I got them Shadow nests instead was night and day.
I also recommend reading up on breeding mechanics. Colors inherit as a range between the parents colors. I stick with fairly close colors for both parents, but another popular strategy is to have close colors in two colors and a really wide range in another, for enough variety to be interesting but enough consistency to have hatchlings that usually look good.
Genes and breeds, however have a sort of "tier" system.
Here's a good guide on the rarity tiers. This can save you a bunch of money and time putting together a pair - you can have one parent with the genes and breed you want, and another with higher tier genes/breed that usually won't pass down.
Oh, and some genes and breeds (and colors) just aren't popular. Like, I have a pair that makes Bogsneaks, but they'd probably sell infinitely better if they were Skydancers instead. I don't care, I like Bogsneaks. At the end of the day, do what makes you happy, and you'll have already "won".