This is a very interesting question, and one that I feel I have enough experience to answer.
I checked your profile and it indicates you are under the age of 20.
First, I'm going to assume that "spastic colon" is the same as what we call here in the U.S. irritable bowel syndrome or "IBS" for short. I've had this problem since I was a toddler, and the ONLY thing that was determined throughout my childhood is that salt-water enemas were the best relief from the pain and discomfort of IBS (back then it was called by yet a third name: colic bowels). That was many decades ago, and I still use saline enemas to treat the symptoms (enemas do NOT cure IBS, by the way, but for some people they provide temporary relief or "remission").
Secondly, given your age, my question is: Are you SURE it's spastic colon and not something else? Have you been through tests such as MRIs, barium enemas, or colonoscopies? If not, then my advice is: DO NOT PUT ANYTHING IN YOUR BOWELS! At least, not until you've been checked out by medical professionals. That means "NO!" to enemas or anything else.
Third, assuming you HAVE been checked out thoroughly, and the final diagnosis is spastic colon, saline (salt-water) enemas MIGHT actually help with the symptoms. Honestly, when I have a flare-up, it is the time that I LEAST want an enema. But, at the same time, I also know that a saline enema, though it initially causes more pain when being administered, will put me into remission (no pain) for weeks and even months before the next flare-up, then it's time to get the bag out again. As an old saying goes, "No pain, no gain" and that's true with my IBS. Otherwise, I suffer. When I'm in remission, however, I can enjoy pain-free enema play solo or with a partner.
Again assuming that you have had a complete medical work-up and it has been determined that your only bowel problem is spastic colon, then I would suggest you try enemas SOLO. That is, don't make it a part of your medfet play until you know for certain it is safe to do. I recommend a small soapsuds enema first to clean out the bulk of the waste in your bowels, then try taking a liter of warm water with approximately one tablespoon (or the metric equivalent) dissolved in the water. Take the enema slowly and allow it to flow gently into the bowels. If the pain increases, stop the enema and evacuate (expel). Wait about an hour and see if the symptoms subside. If they do, try taking another liter of saline solution. If you can take it all, try holding it in for about 20 minutes after the bag is empty. If the pain continues to subside, then saline enemas may be a way to treat the symptoms, and then enjoy the "other" enemas as part of your play.
As always, be careful and be safe!! Avoid putting anything in your enema other than mild soap for the cleansing enema and then saline solutions for the others (Note: If mild soap is too irritating, just do the saline solutions.). Avoid adding alcohol or any other substances. I know some of the "hard liners" here like to put various products in their enemas, but given your young age, I would recommend that you NOT follow that practice.