Hi Ruth,
In your stylesheet, near the top portion, there should be a series of stuff which includes a line " background : #234567 ". The 234567 I just made up, but with the pound sign used, the following numbers are hexadecimal digits (0 to f). The six digits represent three pairs, each for a color - red, green and blue. Higher values are brighter, lower darker. You will presumably need a separate stylesheet to call out for each new color if you want a different look in different galleries.
If all pair values are the same, you will get a gray. #ffffff will be white, #000000 is black. #606060 will give a sort of medium gray. Your photo editor probably has a way to display the numerical values corresponding to colors you click the cursor on. Red and green combined gives some variety of yellow and all the usual tricks. I tried looking at the source for your PBase page, but the stylesheets get pulled in separately when it loads, so I can't see the color setting. I'm sure there are some pages out on the web that get into the codes vs colors, but don't have one linked that I know of. Ah, but Gargoyle is my friend ...
http://www.w3schools.com/html/html_colors.asp. Those are so called "web safe" colors which are less important in these days where most computers can display zillions of colors. We're not typically limited to those now, but it's a starting point.
Graduated color I think would have to be done by having an image file used as the background, but you're beyond my Pbase expertise there. Perhaps Marisa will weigh in. I tend to just learn enough to get by, as such, there may be some tricks available these days that I'm blissfully unaware of.