Is it possible to place buttercream flowers of different colors when frostinf a cake with stabelized whipped cream/ I'm afraid the colors will run.

Well, if they are dark colours you are likely correct. But here is a suggestion, pipe an icing blob in white underneath where your flowers will be placed, one buttercream blob for each flower. That way the white blob will keep the buttercream coloured flowers from sitting directly on the whipped cream. Make sure you get that cake back in the fridge as soon as possible too. Stabilized whipped cream frosted cakes do not sit out for long periods of time, well. So, if you can, make up your buttercream decorations ahead. Refrigerate them or freeze them. Place blobs on the cake, pop on the refrigerated or frozen buttercream flowers and return the cake to fridge.
Hugs Squirrelly Cakes

Thank you for your suggestion. I am making a birthday cake for an elderly
lady who is celebrating her 85th. Most of the ladies who will be there have diabetes so I am making the cake with Splenda and think if I use whipped cream for the icing, that too will cut down on the sugar.

Yes, that is a good idea! There are some recipes for buttercream that use mostly fat and just a bit of sugar. But I guess with diabetes it is also the carbohydrates and the combination of fats and such that all affect the sugar levels, not just the sugar. So I am not so certain that using a high fat ratio icing would really help all that much. It is tough to make desserts and prepare meals for diabetics!
Good luck!
hugs Squirrelly Cakes