You are right that Castilleja indivisa (Entireleaf indian paintbrush) is the common paintbrush you see on the highway rights-of-way growing alongside Lupinus texensis (Texas bluebonnet) and they do look beautiful together. C. indivisa (and other Castilleja spp.) are hemi-parasitic. They can grow without being attached to another plant's roots, but they grow much larger and healthier if they can gain some of their water and nutrients from a neighboring plant. The hemi-parasitic activity of the paintbrush doesn't seem to cause great harm to its victim. (Please see the answer to a previous question about the parasitic nature of C. indivisa.) Bluebonnets, grasses, other wildflowers—just about any other herbaceous plant will do. You can visit our Texas-North Central Recommended page to look for other possibilities. You can use the NARROW YOUR SEARCH option to select criteria to fit your site (e.g., choices under BLOOM TIME or LIGHT REQUIREMENT).
Here are a few that would look good with your paintbrushes and bluebonnets while they are blooming in March, April and May:
Callirhoe involucrata (Winecup) bloom in March, April and May.
Coreopsis tinctoria (Plains coreopsis) bloom in March, April and May.
To keep your garden looking good for a longer period you might want to include some flowers that bloom later in the year such as:
Glandularia bipinnatifida var. bipinnatifida (Prairie verbena) blooms March through December.
Melampodium leucanthum (Blackfoot daisy) blooms March through November.
Gaillardia pulchella (Firewheel) blooms May through August.
Liatris mucronata (Cusp gayfeather) blooms August through December.
Eustoma exaltatum ssp. russellianum (Texas bluebells) blooms June through September.
Early- to mid-fall would have been a good time to plant the seeds outdoors since they do best after experiencing a cold period before spring germination. (Please see the answer to a previous question about planting seeds of C. indivisa.) However, you can do your own cold stratification and start them in containers in a greenhouse. Here is information from the answer to a previous question with extensive information about propagating Castilleja spp.:
According to Tara Luna in "Propagation Protocol for Indian Paintbrush Castilleja species" (Native Plants Journal, Volume 6, Number 1, Spring 2005, pp. 62-68):
"Unlike a true parasitic plant, hemi-parasites are capable of manufacturing their own food and obtaining water and nutrients from soil, but they also form specialized roots known as haustoria roots that attach to the roots of a host plant, therby providing addtional water uptake for the paintbrush plant and possibly some organic and inorganic nutrients. The relationship does not kill the host. Seeds do not require a signal from the host to initiate germination, and they contain endosperm that provides enough energy for the seedling to establish independently. Unless a haustorium root becomes attached to a host root, however, they will decline in vigor, remain stunted in growth, and never flower, or they will eventually die. The degree that a species can grow and develop wthout the host may vary widely between species."
The parasitized plant is frequently a grass. Because of this need for a host plant, a paintbrush plant usually dies if transplanted. Your best bet is to collect seeds and sow them in the fall along with another herbaceous plant. Bouteloua gracilis (blue grama) has been recommended as a host. Ms. Luna recommends using a plant that grows in the same habitat as the paintbrush you are planting. She has used beardtongue's (Penstemon spp.), Carex spp., Polemonium pulcherrimum (Jacob's-ladder), Phlox drummondii (annual phlox), and Lupinus texensis (Texas bluebonnet) as well as small bunch grasses such Festuca idahoensis (Idaho fescue). You should pick something that grows near your paintbrush and collect seeds of them as well as those of your paintbrush.
Collect seeds of your paintbrush as soon as they are mature and ready to be released from the plant. You will have to watch the plants carefully to be sure that you don't lose the seeds when they mature, but also don't collect them too early. Seeds that aren't mature when collected won't germinate. You can sow them directly in the area you want them at the time when they would naturally be released and hope for the best. They obviously have been doing this for millenia on their own and have been successful. Be sure to sow your host plant(s) at the same time.
Ms. Luna, however, uses cold stratification (30 to 150 days, depending on the species—generally, longer times for higher altitude or colder area species) and then sows them in containers. She says that annuals and semi-annuals will germinate without cold stratification, but the percentage of germination will be higher with cold stratification. Here is a general summary of what she does:
1. Soaks seeds in water for 4 to 8 hours. Pours off water and debris.
2. Places seeds between moist blotter paper and stores in an open plastic bag in refrigerator at 33-36°F for required time, checking weekly to see that moisture is evenly distributed.
3. Sows the seeds shallowly into containers and covers them lightly with perlite mulch. She sows the host separately. Germination is 69-77° F daytime temperatures with 50-60° F night temperatures.
4. After the paintbrush plants have 4-6 sets of true leaves (4-6 weeks after germination) they are combined with the host plant seedlings in the same containers.
5. At about 16 weeks they are ready for outplanting. Perennials usually bloom the second year after germination. The annuals and biennials flower the first year after germination.
You can also find instructions for C. indivisa in Tom Clothier's Annual/Biennial Seed Germination Database.