A few months ago I was collaborating with a friend of mine who is a designer. He was putting together a nifty branding guide in InDesign, but we realized that every time he used a transparency effect, all his RGB colors turned to dull-as-doldrums CMYK. I've made this little graphic to illustrate the difference. The same color swatches are displayed in the CMYK color space on the left and the RGB color space on the right.
1. Go to Edit.
2. Go down to Transparency Blend Space.
3. Select Document RGB.
Your colors should be back to their bright and shiny RGB goodness.