what is the rule in these cases?

[Steve Hajdu]

Roger sort avec une jeune fille aux yeux...
a: bleues claires; b: bleu clair; c: bleus clairs; d: bleue claire

Valerie s'est acheté deux vestes...
a: vert foncé; b: verts foncés; c: vertes foncées; d: verts foncé

Thanks for the help

[Frank]

Roger sort avec une jeune fille aux yeux bleus clair.

Valérie s'est acheté deux vestes vert foncé.

Colors are extremely tricky in French.

But clair and foncé after a color name are always invariable. And the presence of clair or foncé also implies that the color name is invariable.

Les yeux sont bleu clair
Les yeux sont bleus

Another case where a color name is invariable is when it's the name of an object (usually a fruit or a flower).

The most frequent exemples are probably marron and orange.

Les yeux sont marron
Les cheveux sont orange

[Neil Coffey]

Frank -- many authors actually advocate making the colour name invariable too in this case, so you'd write e.g. aux yeux bleu clair. In fact, I don't think I've seen a teaching grammar in the UK mention any other possibility. (Actual usage varies: some speakers apparently make the colour name invariable, but others make it agree.)

Edit: sorry, I think I misread your post because I saw you'd made the agreement in the first sentence.

[Frank]

Oops sorry for the confusion, this extra "s" in the first sentence was a typo.