NO, you can only work in Adobe 1998 RGB and sRGB which is suitable for Internet

RGB has a larger gamut than CMYK. In order from small to large: sRGB, AdobeRGB, ProPhoto RGB. The largest of the CMYK gamut is GRACoL.

From the edit menu in Adobe Photoshop CS4, your can select color settings . Their are different settings for web designer and photographers . For instance , a web designer can use color setting sRGB .

RGB is color model and is used to describe colors using numbers and to determine number of possible colors. You can change color model from Image > Mode > choose color model. RGB is suitable for screen and different devices including internet. Other color modes are CMYK suitable for professional printing, Lab, Grayscale.

Although RGB is mainly for devices and screens it is recommended to work in this color mode before converting to CMYK.

Different color spaces within color mode can contain different range and number of possible colors in document (image). RGB color mode have different spaces like sRGB which can contain less colors then Adobe RGB but is supported by most browsers while browsers do not support Adobe RGB color profile, so when you want to post images on internet or send them on email first convert to sRGB profile (space) from File > Save for Web & Devices (turn on Convert to sRGB) or Edit > Convert to Profile > RGB > sRGB IEC61966-2.1.

ProPhoto RGB is largest color space and is designed for feature devices and printers...

what color problem you can edit color settings from Edit-Color Settings and choose North America General Purpose 2 if you preparing pictures for web,and Working Spaces RGB :sRGB IEC61966-2.1.If you have problem with colors on your picture then try Levels from pallete Adjustments.You can choose RGB,R,G,or B from drop-down list above histogram where RGB is composite and R-Red color,G-Green,B-Blue , play arround with sliders below histogram to accomplish colors