Using the Stone colour, cast on 64 sts and divide them onto four needles, 16 sts each. The beginning of the round is between needles I and IV. Work ribbing in the round for 3 cm and on the last round increase 1 st on each needle = 68 sts, 17 sts each.
Begin the colourwork pattern from row 1 of the chart. Keep working the pattern repeat to end of round. Work rows 2–85.
On rows 30, 35, 40, 45, 50, 55, 60, 65 and 70 decrease 1 st at the beginning and end of the round by k2tog (first 2 sts and last 2 sts). 50 sts on the needles after the decreases. Redistribute the sts so that you have 13 sts on needles I and IV and 12 sts on needles II and III.
When you have worked row 85 of chart I, begin the heel flap. Using the Sand colour, knit the sts on needle I onto needle IV (= 26 sts). Leave the other sts on hold. Turn work and begin the reinforced st pattern:
Row 1: slip 1, purl to end. Turn work.
Row 2: *slip 1, knit 1*, repeat *–* 12 more times.
Repeat rows 1–2 a total of 12 times (= 24 rows).
Work another WS row and begin turning the heel: keep working the reinforced st pattern as before. On the heel flap, work a RS row until 9 sts remain at the other end. Work the skp decrease (= slip 1 knitwise, knit 1, pass the slipped st over) and turn work. Slip 1, p8, p2tog, turn work. Slip 1, work until 8 sts remain, skp. Continue in the same manner, decreasing sts on the sides with 10 sts in the middle. When only the middle sts remain, distribute them onto two needles, 5 sts each. Continue the colourwork pattern from row 86 of chart II with the Sand yarn.
Pick up 12 sts from the left edge of the heel flap + 1 st from between the needles. Knit the picked-up sts through the back loop onto needle I. Knit the sts on needles II and III. Pick up 12 sts from the right end of the heel flap + 1 st from between the needles and knit the picked-up sts through the back loop onto needle IV.
With these 60 sts, work rows 87–130 of chart II. At the same time begin the gusset decreases: k2tog at the end of needle I, skp at the beginning of needle IV. Work the decreases in the same manner on rows 89, 92, 95, and 99. 50 sts now on the needles (= 13, 12, 12, and 13 sts).
On row 124, decrease 1 st on needles I and IV = 48 sts, 12 sts each. Begin the toe decreases on row 125: k2tog in the middle and end of each needle = 40 sts. Work 4 rounds without decreases (rows 126–129) and then repeat the decreases on row 130 = 32 sts. Use the Stone yarn for the rest of the sock. Work 3 rounds without decreases and then repeat the decreases = 24 sts. Work 1 less round without decreases after each decrease round. Continue decreasing in the established manner until 8 sts remain. Break the yarn, pull it through the sts, and securely weave in.
Knit the other sock to match.
Steam the socks lightly.