Normally I whip up some cream at home for cake icing … I whip the whole cream and then use it little my little to add coloring …
I use wilton gel colors … I have noticed the cream is smooth and soft when its white and as soon as I start mixing color in the cream it becomes thick and buttery … When I mix colors I dont do the whipping action , I just mix it slowkly with a spoon …
Even though you are not whipping it when you mix the colour I guess more air still gets added. Try adding colour a little earlier so it doesn't get over whipped. Although, if you are folding in the colour it really shouldn't be becoming buttery.
Why don't you try adding the colour to the whipping cream before you start whipping it? It might even be a little easier to incorporate the colour in it than doing so afterwards.
saks , I keep the cream white while whipping it , coz then later on it can become really difficult to whip up tiny bits of cream in each color i want to use .. normally its like whipping the whole lot i need .. and then add colors as required ...