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

Closes #76

Reviewed-on: #77

backend/colour_test.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestConvertStringToColourWithSign(t *testing.T) {
+	colour, err := ConvertStringToColour("#123456")
+	require.NoError(t, err)
+	require.Equal(t, 0x123456, colour)
+}
+
+func TestConvertStringToColourWithoutSign(t *testing.T) {
+	colour, err := ConvertStringToColour("123456")
+	require.NoError(t, err)
+	require.Equal(t, 0x123456, colour)
+}
+
+func TestConvertStringToColourWithSignThreeDigits(t *testing.T) {
+	colour, err := ConvertStringToColour("#ABC")
+	require.NoError(t, err)
+	require.Equal(t, 0xA0B0C0, colour)
+}
+
+func TestConvertStringToColourWithoutSignThreeDigits(t *testing.T) {
+	colour, err := ConvertStringToColour("ABC")
+	require.NoError(t, err)
+	require.Equal(t, 0xA0B0C0, colour)
+}