Herbs, annual, herbage usually drying dark. Stems erect or ascending, 10–140(–220) mm, glandular-puberulent to glandular-pubescent. Leaves usually cauline, relatively even-sized; petiole absent; blade narrowly elliptic to narrowly oblanceolate, linear-lanceolate, or linear, (4–)7–23(–34) × 1–5(–8) mm, margins entire, plane, not ciliate, apex acute, surfaces glandular-puberulent. Pedicels 1–3(–4) mm in fruit. Flowers 2 per node, or 1 or 2 per node on 1 plant, chasmogamous. Calyces symmetrically attached to pedicels, not inflated in fruit, (3–)4–8(–10) mm, glandular-puberulent, tube not strongly plicate, lobes triangular, subequal, apex acute-apiculate, ribs narrow, darkened, purplish, thin, not strongly raised, intercostal areas pale green. Corollas: (a) magenta, darkening toward mouth and within tube, often nearly obscuring dark longitudinal stripes within, throat floor or at least palate ridges yellow, or (b) yellow, usually with maroon stripes in throat extending onto lobe bases, sometimes with maroon blotches on adaxial lateral walls of throat, colored palate ridges ending in throat, tube-throat (10–)13–18(–20) mm, limb 10–19 mm diam., not bilabiate. Anthers included, ciliate. Styles glandular-puberulent. Stigmas included, lobes equal. Capsules (4.5–)6–10(–13) mm. 2n = 16.
Phenology: Flowering May–Sep.
Habitat: Disturbed areas, water runoff areas, granitic soils, edges of granite outcrops.
Elevation: (1200–)1500–3300 m.
Diplacus bicolor is endemic to Fresno, Kern, and Tulare counties in the southern Sierra Nevada. The combination D. whitneyi (A. Gray) G. L. Nesom is illegitimate, as Mimulus whitneyi A. Gray 1886, is a replacement name based on Eunanus bicolor A. Gray 1868.
Diplacus bicolor, D. bigelovii, D. constrictus, D. graniticola, D. layneae, and D. thompsonii appear to be closely related species, sometimes intergrading where sympatric. Plants of each species often produce flowers at all nodes and have dark magenta corollas with nearly regular to weakly bilabiate limbs and villous vestiture. Specimens of each species dry to a dark color.