I am trying to make a Monet waterlilies cake, and I think the effect I'm going for is most similar to this photo I found: http://www.acaketoremember.com/images/waterlily_cake_3.jpg

So now I am trying to figure out the easiest way to obtain the impressionist/painting look, and be able to have the colors be subtle and pastel-like, rather than too dark or bold.

Alright so my idea was the make various thin buttercream frostings in the different colors I'd want (white, blue, violet, pink, green), and apply the colors to the crumb-coat in a piping method (tips would be probably just round...4, 5, whatever... and then after I've gotten the cake pretty covered in a scrawled/rough color placement, take a warm spatula and glide over them so they look like an impressionist painting-- all running into each other and blended.

But now I'm wondering if that would really work. There's also the option of actually "painting" which I've never done.

I would prefer to use butter cream rather than fondant, which I know might not make as great of a "canvas" for painting.

Any ideas?

I can tell you how I do it. I use fondant. I use airbrush colors mixed with everclear and a brush.

http://acmecakes.com/images/IMGP0216.JPG

The clothing is buttercream.

HI - I'm not sure if this will help since you specifically said you want to use buttercream, but I thought I'd post since I just did a cake that I tried to get the Monet Water Lilies look-- so if you decided to use fondant, this is what I did.
I tinted marshmallow fondant teal, which tastes GREAT by the way (you could use a light blue like the color in the picture you are trying to make). Then, I mixed green gel coloring and a little Vodka on a plate and lightly dipped a small sponge into it and sponged little patches of "lilly pads", not worrying about making it smooth and uniform, dark color blotches are good. I let it dry a little, then I took a soft paint brush lightly dipped in Vodka and rubbed the blotches of color to smear and blend it.
I made little flowers by flattening small balls of regular fondant (so they'll dry hard, I used Wilton ready made fondant and tinted it). I shaped them over a ball tool and let them dry before using Vodka to "glue" them on the cake.
I am SO not an expert here, and I'm not too good with buttercream.
Good Luck!

just wanted to say I LOVE LOVE LOVE that waterlillies cake that you're using as your inspiration!! I LOVE monet....that said...........I think you're onto the right idea with watering down your icing that's already the chosen colors you want........however, instead of piping it on...just thin it and use a paint brush and paint it on. I did that with the cake (the trees were all painted on w/ loose buttercream) after about 10-15 minutes the frosting sets right back up to a normal consistency and will even crust! I really think that would give you the best and most realistic look. THen you will have the texture an actual painting would have as well vs. having it all be super smooth like fondant would be.

HTH