What word did Middle English have in place of “light" as in: “light blue”, “light green” etc.?
Solution 1:
It seems that you could use Fair, Fade or Faint (there might be others)
Fair
and its other forms
fair (adj.) Also (early) faȝer, faier, fei(e)r, vair, fare, fer(e, war, fæger, fægerra, fægrost
According to the Middle English dictionary from Umich
(a) Light, bright, or shining (as opposed to dark); (b) of persons: light of complexion or color of hair and eyes; fair.
It seems to have been used with color (other than applied to skin or complexion):
Found at least one reference as cited in Altenglische legenden ... Horstmann, Carl, ed. b. 1851.
þat þis on schal beo fair blu cloth
...
Faint
Of things, actions, functions, etc.: weak, feeble; impaired, poor; pale (color)...
...
Fade
Of color or things having color: lacking in brilliance; pale, discolored, dim; dull; fade-heued.
(Fade-heued meaning "light-colored")
>c1300 SLeg.Mich.(LdMisc 108) 672: Ho-so hath of þe eorþe mest, he is..Of fade [vr. vad] colur.
...
As for your example of "whyte Russet" I think russet is used here in the sense of
the same way Whites can be used for "clothes usually of white color" in modern sense, Russet is used by analogy for the piece of cloth usually of the color Russet or the fabric material itself with which one makes russets.
See here in The Stonor letters and papers, 1290-1483; ed. for the Royal historical society, from the original documents in the Public record office, by Charles Lethbridge Kingsford, for Russet used with the color Black, for example, in the sense of piece of cloth or fabric material.
a ȝerde off blake russet karyssey to make Richert Baron a dublet
a payre off hosyn off russet karyssey,
Item a gowne of russet furryd wt blakke,
another example of russet used in the sense of fabric.
And is as gladde of a goune · of a graye russet /
So the answer is I don't think whyte russet mean a light russet (color).
Solution 2:
From about 1300 is Lenten ys come with loue to toune
(alternative source)
The rose rayleth hire rode;
The leues on the lyhte wode
Waxen al with wille.
which has been translated as:
The rose begins to blush;
The leaves in the light-green wood
All unfurl gladly.
Other translations exist such as that of Aniee Jeong
An early modern English example is from Parthenophil and Parthenophe. Sonnettes, Madrigals, Elegies and Odes
There in a mantle of light green,
(Reserved, by custom, for that day)
Parthenophe, they did array !
And did create her, Summer's Queen !
And Ruler of their merry May !
Barnabe Barnes, May 1593.