Hello and thank you for your question about hollyhocks changing flower color.  Unfortunately, there is probably no guaranteed way to get "back" the dark purple flower color other than purchasing new plants or seeds of that color.  Hollyhocks are not reported to change flower color like hydrangeas, based on soil acidity.  However, many growers of the deep colored varieties report that they gradually fade to pink.  Hollyhocks are also usually biennial in most climates, so it is possible that the pink hollyhock is actually growing from a seed that was produced by the original dark flowered plant before it died.  The seedlings would not necessarily be the same color as the parent.  Frequently, unusual colored flowers require very specific conditions of temperature, light exposure, and soil type to remain true to type.  They slowly lose that color if all those special conditions are not met.  The cool spring weather may have had a hand in the color change this year, so if you can tolerate the pink this year, it is possible that the flowers may be darker next year if the spring weather is different.