Yellow may be expressed with flavus (a pale version), luteus or lutea, as in the charming perennial Corydalis lutea, top photo), and citrinus (lemon-colored).
Red is rubrum; rosy-pink is roseus.
Purple is simple: just say purpureus. If it’s very dark, it might be atropurpureus.
White is albus; black is nigrum.
Silver is expressed as argenteus; gold is aureus.
As for good old green, when it’s noted it might be viridis (or sempervirens in the case of evergreen).
There are various words for blue, including azureus (a sky-blue color) and caeruleus (somewhat darker).
Variegated leaves or flowers are sometimes labeled variegatus, but might also be called pictus (which means painted, and is used to indicate bright coloration of other kinds, too).
If a plant is graceful or slender, it might be designated gracilis. If it is globe-shaped, globosus might be more appropriate. A pyramid, not surprisingly, is often expressed by pyramidalis.
A shrubby plant might be labeled fructicosus or frutescens. Upright and columnar? Look for the words fastigiatus or columnaris. Downright narrow, with nearly parallel sides: linearis.
A dwarf plant might be nanus or pumilus; a creeping one, repens; one flat on the ground, prostratus or procumbens. If they spread in a straggly manner, the specific epithet divaricatus is a possibility. If instead the plant climbs, it could be called scandens.
Pleated leaves might be indicated by the word plicatus. Woolly ones are often labeled lanatus.
Mollis means soft (because the plant is covered with soft hairs); glaucus plants are coated in what’s called bloom (a fine white powdery coating).
If the surface glistens, it could be called fulgens. If it’s spiny, spinosus is a more appropriate epithet.
Spots might indicated by the word punctatus.
Inodorus means a plant has no fragrance. Aromaticus and fragans mean that it does. But so do pungens (pungent), odoratus (sweet-smelling), and foetidus (fetid, or stinking).
Some epithets, such as praecox, mean simply early. Spring interest is expressed by vernalis; summer by aestivalis; fall by autumnalis, and winter by hyemalis.
Plants from wet places are often called palustris (or aquaticus, if they actually live in water). Those from rocky areas may bear the specific epithet saxatilis; if sand was in their background, arenarius is the word. Woodland denizens may be sylvaticus or sylvestris; those from above the treeline are alpinus.