Determine the color of a chess square

Python 2, 41 38 bytes

print'ldiagrhkt'[int(input(),35)%2::2]

3 bytes thanks to Mego for string interlacing

Takes input like "g6". That's light and dark intertwined.

GS2, 17 15 bytes

de♦dark•light♠5

The source code uses the CP437 encoding. Try it online!

Verification

$ xxd -r -ps <<< 6465046461726b076c696768740635 > chess.gs2
$ wc -c chess.gs2 
15 chess.gs2
$ gs2 chess.gs2 <<< b1
light

How it works

d               Add the code points of the input characters.
 e              Compute the sum's parity.
  ♦             Begin a string literal.
   dark
       •        String separator.
        light
             ♠  End the string literal; push as an array of strings.
              5 Select the element that corresponds to the parity.

Hexagony, 34 32 bytes

,},";h;g;;d/;k;-'2{=%<i;\@;trl;a

Unfolded and with annotated execution paths:

^{Diagram generated with Timwi's amazing HexagonyColorer.}

The purple path is the initial path which reads two characters, computes their difference and takes it modulo 2. The < then acts as a branch, where the dark grey path (result 1) prints dark and light grey path (result 0) prints light.

As for how I compute the difference and modulo, here is a diagram of the memory grid (with values taken for the input a1):

^{Diagram generated with Timwi's even more amazing Esoteric IDE (which has a visual debugger for Hexagony).}

The memory pointer starts on the edge labelled row, where we read the character. } moves to the edge labelled col, where we read the digit. " moves to the edge labelled diff where - computes the difference of the two. ' moves to the unlabelled cell where we put the 2, and {= moves to the cell labelled mod where we compute the modulo with %.

This might be golfable by a few bytes by reusing some of the ;, but I doubt it can be golfed by much, certainly not down to side-length 3.