Desteñirse vs. descolorarse
While both desteñirse and descolorarse translate to "to fade" or "to discolor," they describe different processes. Desteñirse refers to the loss of artificial dye, often involving the color running or bleeding, whereas descolorarse refers to the loss of color intensity due to external factors like light or time.
Desteñirse
B2This verb comes from teñir (to dye). It specifically refers to an object losing the dye or pigment that was chemically applied to it. It is most frequently used in the context of laundry, implying that the color is washing out or bleeding onto other clothes.
No laves esa camisa roja con agua caliente o va a desteñirse.
(Don't wash that red shirt with hot water or it is going to fade.)
Mis jeans nuevos se destiñeron y me mancharon las piernas de azul.
(My new jeans faded and stained my legs blue.)
Si usas blanqueador en esa tela, seguramente se destiña.
(If you use bleach on that fabric, it will surely lose its dye.)
Ten cuidado, el traje de baño puede desteñirse en la piscina por el cloro.
(Be careful, the swimsuit might fade in the pool due to the chlorine.)
Separé la ropa blanca para que no se destiña la oscura sobre ella.
(I separated the white clothes so the dark ones wouldn't bleed onto them.)
Descolorarse
B2This verb comes from color. It signifies becoming pale or losing vibrancy due to environmental causes. It is primarily used when the sun, weather, or old age causes an object (not necessarily fabric) to lose its original brightness.
La pintura del coche se descoloró por estar estacionado bajo el sol.
(The car paint faded because it was parked under the sun.)
Las fotos antiguas en la pared se han descolorado con los años.
(The old photos on the wall have faded over the years.)
No dejes los juguetes de plástico afuera, se van a descolorar.
(Don't leave the plastic toys outside, they are going to become discolored.)
El póster se descoloró hasta quedar casi blanco.
(The poster faded until it became almost white.)
Las hojas de los árboles se descoloran antes de caer en otoño.
(The tree leaves lose their color before falling in autumn.)
Summary
Use desteñirse when talking about fabrics or dyed items losing their color, especially during washing or due to water chemicals. Use descolorarse when talking about any object becoming pale or lackluster due to sun exposure, weather, or the passage of time.