Jen, a nice plant for blue flowers all season is Evolvulus, commonly called Blue Daze. It's a low-growing spreading clump like plant - maybe 10in. tall and 2ft wide at maturity - with intense blue flowers about an inch across . Easy to grow and available everywhere. Wait! running outside to snap a picture of mine. (forgot to note, the flowers close in the late afternoon)
The other really wonderful blue is Plumbago which will grow to be a large shrub but you can prune it to any size that suits you. Again, easy to grow and available.
For yellow, Hmmm I love yellow but not the really bright, chrome yellow of a lot of flowers. Golden yellow or pale pastel yellow are more my preference. In fall/winter/spring I grow nasturtiums (annual from seed) that are pale yellow. They give up in the heat of summer though. For most of the year color, Bulbine is excellent (it's actually a type of succulent) but I don't have any right now. I have Spathoglottis (another ground orchid) that are pale yellow with other colors mixed in though. They need some shade in the afternoon.
Nasturtiums
Spathoglottis
These yellow dwarf cannas are a beautiful color with subtle pink mottling on the petals, love the sun and heat and can take any amount of water you (or mother nature) give them. They're always available at Lowe's and will keep on blooming almost year round if you cut off the spent stems to keep them growing new ones.
Nasturtiums
Spathoglottis
