1. First, follow the colored lines to see which number boxes go with which symbol 
(+ * - /)
column 1 row 1 and column 1 row 2 = blue = +
column 1 row 3 and column 1 row 4 = green = *
column 2 row 1 and column 2 row 4 = red = /
column 2 row 2 and column 2 row 3 = purple = -

Now fill in the boxes with the following numbers:
2<3<4
6<7<8
3<4<5 (I think the symbols in this row is supposed to be switched. box<4<box instead of what's shown in the game box>4>box)
4<5>4

Now going left to right (+ * - /), follow the lines and do the math on those boxes:
+ = 2+6 = 8
* = 4*3 = 12
- = 8-5 = 3
/ = 4/4 = 1

Enter the code: 81231

2. Proceed to the next level!