Best tip I have is the keyboard shortcut Opt-Cmd-A (Mac). Press this after selecting any layout item and all items of the same type are selected. (For your purpose, start by clicking on a field.) Then in the inspector select the color you want and it will be applied to all the fields on that layout. If its an odd color you can place it in the adjustable pallette at the bottom of the color picker. Repeat for all layouts. It shouldn't be too bad for 50 layouts unless you have a lot of fields that are grouped or locked, or that use conditional formatting. This keyboard shortcut doesn't apply in those conditions until you manually remove the grouping, locking or conditional formatting.
Another approach is to download the free Clipboard Explorer graciously supplied by Dracoventions <http://www.dracoventions.com/products/2empowerFM/family/clipboard.php> to copy, edit and repaste your layouts. You will want to notice the location of your layout contents when you copy it, and then move it back to the same place after pasting. And you may have to adjust the length of your layout if the Paste forces it to extend.
For this particular situation I don't know if this second approach is worth the learning curve, but it is a handy approach to a variety of "bulk updates."
also, the Format Painter works well. It can be "locked", so you just click around the layouts to "paint" your color
someday, we will have a proper editable CSS for the entire solution ...
> I have about 50 layouts and my boss wants the fields background colour in all layouts to be changed slightly