Add support for colour attribute on allowances in backend (#77)

Closes #76

Reviewed-on: #77

backend/colour.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+)
+
+func ConvertStringToColour(colourStr string) (int, error) {
+	if len(colourStr) == 0 {
+		return 0xFF0000, nil // Default colour if no string is provided
+	}
+	if colourStr[0] == '#' {
+		colourStr = colourStr[1:]
+	}
+	if len(colourStr) != 6 && len(colourStr) != 3 {
+		return 0, errors.New("colour must be a valid hex string")
+	}
+	var colour int
+	_, err := fmt.Sscanf(colourStr, "%x", &colour)
+	if err != nil {
+		return 0, fmt.Errorf("invalid colour format: %v", err)
+	}
+	if len(colourStr) == 3 {
+		r := (colour & 0xF00) >> 8
+		g := (colour & 0x0F0) >> 4
+		b := (colour & 0x00F) >> 0
+		colour = (r << 16 << 4) | (g << 8 << 4) | (b << 0 << 4)
+	}
+	return colour, nil
+}