Using the smaller needles and the main colour, cast on 60(80)88 sts and divide them onto four needles, 15(20)22 sts each. The beginning of the round is between needles I and IV.
Begin ribbing in the round on row 1 of chart I. Work the 4 st pattern repeat to end of round. Work rows 2–5 of the chart. Work row 6 as follows: work 2 sts, work the 4 st pattern repeat 15(19)21 times, work 3 sts. The beginning of the round is marked red on the chart: move the last st onto a free needle without knitting it (= 1st knit st of row 7) and work to end of row 7. Work rows 8–17. Note: Row 14 is worked in the same manner as row 6.
Switch to the larger needles and work 2 rounds of stockinette st.
Begin the colourwork pattern on row 1 of chart II(III)IV. Work the 2 st pattern repeat to end of round.
Size 30: Work rows 2–61 of the chart. Note: On rows 18, 24, 34 and 43, decrease 1 st at the beginning and end of the row. On row 57, decrease 1 st on each needle.
Size 38: Work rows 2–83 of the chart. Note: On rows 23, 28, 43, 47, 51 and 73, decrease 1 st at the beginning and end of the row. On rows 33, 71 and 77, decrease 1 st on each needle.
Size 43: Work rows 2–97 of the chart. Note: On rows 16, 26, 32, 37, 51, 55, 59 and 80, decrease 1 st at the beginning and end of the row. On rows 46, 63 and 86, decrease 1 st on each needle.
48 sts on the needles after the decreases. Distribute the sts onto four needles, 12(14)15 sts each.
When you have worked row 61(83)97 of chart II(III)IV, begin the heel flap: using the main colour, knit the sts on needle I onto needle IV = 24(28)30 sts. Turn work and leave the other sts on hold.
Using the contrast colour, work the reinforced st pattern:
Row 1: purl the WS row. Turn work.
Row 2: *slip 1 st purlwise (yarn in back), k1*, repeat *–* 11(13)14 more times.
Row 3: slip 1 st purlwise (yarn in back), purl to end of row. Turn work.
Work rows 2–3 a total of 12(14)16 times =24(28)32 rows.
Begin turning the heel: continue working the reinforced st pattern as established. Work until 9(10)11 sts remain at the left end of the heel flap. Work the skp decrease (= slip 1 st knitwise, k1, pass the slipped st over) and turn work. Slip 1 st, p6(8)8, p2tog. Turn work. Slip 1 st, work until 8(9)10 sts remain, skp. Continue in the same manner, decreasing sts at the sides with 8(10)10 sts in the middle. When only the middle sts remain, divide the heel flap sts onto two needles, 4(5)5 sts each. K4(5)5 to move the yarn between needles IV and I.
Using the main colour, k5 on needle I. With a free needle, pick up 12(14)16 sts from the left end 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 onto needle IV 12(14)16 sts from the right edge of the heel flap + 1 st from between the needles. Knit the picked-up sts through the back loop and k4(5)5. 58(68)74 sts on the needles.
Switch to the smaller needles. Knit 1 round and begin the gusset decreases: k2tog at the end of needle I, skp at the beginning of needle IV.
Continue the colourwork pattern from row 62(84)98 of chart II(III)IV and repeat the decreases on every 3rd round. Keep repeating rows 62–73(84–95)98–109 of chart and working the decreases until 12(14)16 sts remain on each needle.
When the sole measures 14(18)21 cm or the sock covers your fifth toe, work stockinette st with the main colour for the rest of the sock.
After 1 round, begin the toe decreases: k2tog at the midpoint and end of each needle = 40(48)56 sts. Work 4(5)6 rounds without decreases, then repeat the decreases as established = 32(40)48 sts. Work 3(4)5 rounds without decreases, then repeat the decreases = 24(32)40 sts. 1 less round without decreases after each decrease round. Continue in this manner until 8 sts remain. Break the yarn, pull it through the sts and securely weave in.
Knit the other sock to match.
Lightly steam the socks.