If you're going to use a background colour in Photoshop to simulate the paper, make sure it's on a separate layer below your main image layers, and make your image layers about 85-90% opacity. You want to see what the paper is contributing to the image, but you absolutely do not want the colour to be included in the final image (turn off the layer visibility or delete the layer before printing, and don't forget to make your other layer(s) 100% opaque before printing)—otherwise you'll be printing the cream colour onto cream-coloured paper and wasting a lot of yellow and magenta ink.
There is another option, and that's to turn on "soft proofing" while you're working (it should be under the View menu as "Proofing setup"). There is an option in the soft proofing module to simulate paper colour, but it is kind of limited in the range of paper tones it can apply, and it usually means creating (or obtaining) a printer/paper profile for the printer and paper you are using. You should be able to simulate a cream-coloured paper well enough, but it'll be useless with, say, a hot-pink fluorescent paper. The advantage to the soft proofing method is that there is nothing to forget when going from screen to print—the paper colour only exists on screen to show you the effect of the paper, and never becomes part of the image file.
