I think dans la pénombre would do
Ombre is shadow
Pénombre means in quite-shadow, thus in very slight light, a bit like twilight
Another expression in french is entre chien et loup(between dog and wolf), the slight time between late afternoon and early night when the sunlight begins to fade.
If you mean "in the light subtly" literally, the translation is "dans les lumières subtilment". This would be used in the sentence: "He walked in the light subtly".
To say "light" as in a light color in French, you can use the word "clair."
a light skin (in the sense of "fair complexion") is 'peau claire' in French.
In French, "light blue" is translated as "bleu clair."
Lampe
leger = light
The English phrase, "writing with light", is "écrire avec la lumière" in French.
Ampoule - light bulb.
Feu vert
petite lumière
Coca cola diète
Les lampes sont eteintent
a light switch is 'un interrupteur' (masc.) or familiarly 'le bouton'