The colours "orange" (orange) and "brown" (marron) do not change form (are invariable), regardless of the gender or number of the word they modify. Also, any modifier attached to any colour word renders it invariable. For example, the colour "light blue" (bleu clair) is invariable.
Which colors vary or not according to gender and numbers in French is actually a quite complicated matter, with several rules and exceptions. A complete reference can be found here : http://fr.wikipedia.org/wiki/Accord_des_adjectifs_et_des_noms_de_couleur And here is a simplified version : http://pourpre.com/langue/accord.php
In any case, you would never say « bleu légère », even if the noun it's referring to is feminine. You also can't say « bleu léger » (or « léger bleu ») just like that, it's not a color ; the corresponding color is « bleu clair » (and not « clair bleu » -- just to be precise ; also not « bleus clairs / bleue claire / bleues claires »).
But I do think that in some contexts, it would be ok to use the expression « léger bleu » or « bleu léger » when speaking about some kind of pale, somehow not well-defined blue (I hope I'm being clear), with an article : « elle portait un foulard d'un bleu léger », « le léger bleu de sa jupe », these kind of things. For all I know, it might be of recent use only and inspired by English, I can't say.
As a sidenote, this is not to be confused with « a / some / the light bruise(s) », which in French would translate into « un or des or de / le or les léger(s) bleu(s) ».
Also, just to try and be complete : « bleu pâle » (pale blue) does exist, as well as « bleu foncé » (dark blue).
About « vert pomme », it is actually the opposite, in some way : the noun « pomme » is used as some sort of second adjective to qualify the shade of the color « vert », which is the "main adjective", in a sense ; both adjectives actually forming a single compound adjective, and thus it is invariable : « des yeux vert pomme ».
As a sidenote, more tricky is the fact that some color adjectives - even a few "basic" ones - actually come from nouns : « orange », of course, but also « marron » (brown ; the noun « marron » meaning "chestnut") and « rose » (pink) for example. As such, they should be invariable. But... of course there are exceptions, and some of them - most notably « rose » - are not ! Also, since most people don't know this rule, it is not considered a big mistake to write « marrons » or « oranges » even when they are adjectives.
More to the point, to try and answer your question : I'm not a grammarian nor a linguist, so I can't say absolutely for sure ; but from my point of view, I wouldn't formulate it that way. Whether it is formed from two adjectives - « bleu clair » - or an adjective and a noun - « vert pomme » -, it becomes, as I said, a single compound adjective.
Which doesn't mean that in another context, it can't be used as a noun : "pale blues" translates to « des bleus pâles », for example. But then the rules become even more complex : if the compound adjective is made from an adjective and a noun, and that you use this compound adjective as a noun, then only what originally already was an adjective varies. To try and make that more clear : you may say a hat is a particular shade of yellow, similar to the color of straw ; you would say « un chapeau jaune paille », plural « des chapeaux jaune paille » (compound adjective, invariable). But if you want to talk about that precise color and say you like it, you would say « J'aime le jaune paille », or, in plural form (let's admit for the sake of the argument that there might be different shades of that color) « J'aime les jaunes paille » ; in that precise latter case, the color « jaune », which is originally an adjective, is used as a noun - hence the fact that it varies - while « paille » (straw), in a sense is the opposite : originally a noun, but somewhat used as an adjective, and hence invariable - even if, to be really precise, they both form a single compound name.