plistxcode
If I open a .plist file in TextEdit, all the properties with type "data" are displayed in Base64 encoding, for example
<data>SGFja2ludG9zaA==</data>.
Xcode shows these values in some other format, like
<54657374 56616c75 65>. What is this format called, and how can I convert it to ASCII?
Use RCDefaultApp prefpane. You can get it from http://www.rubicode.com/Software/RCDefaultApp/
Open it, select UTIs and scroll down to com.apple.xcode.project. 
On the right you can change which filetype is associated to which application or disable the association completely.
Edit: another alternative is Magic Launch, which allows the same kind of file association overrides.
Open it, drag an Xcode project file into the pane on the left, set the Default Application on the right to the appropriate version of Xcode.
bplist means binary property list, which can be converted to XML with plutil -convert xml1. Inside each plist for a color is another data key for the fractional RGB values.
/usr/libexec/PlistBuddy -x -c 'Print "Window Settings":"My Theme"' ~/Library/Preferences/com.apple.Terminal.plist|tr -d '\n\t'|grep -o '[^>]*</key><data>[^<]*'|while read l;do echo ${l%%<*} $(base64 -D<<<${l##*>}|plutil -convert xml1 - -o -|awk '/<data>/{getline;print}'|tr -d '\t'|base64 -D);done
The output looks like this:
ANSIBlueColor 0.4769933663 0.4769933663 0.9314516129
ANSICyanColor 0.2666655827 0.8165705831 0.8588709677
ANSIGreenColor 0.428833897 0.8508064516 0.490967087
ANSIMagentaColor 0.9072580645 0.4499707336 0.9072580645
ANSIRedColor 0.9072580645 0.508503512 0.508503512
ANSIYellowColor 0.9072580645 0.9072580645 0.3914379553
CursorColor 0.9998760223 0.999984026 0.999786377
SelectionColor 0.3899414837 0.4639441073 0.5917825699
TextBoldColor 0.9441435337 0.4102420509 0.427282244
TextColor 1 0.99997437 0.9999912977
Best Answer
The data is just a hex dump of the decoded base64 encoding. To convert to ASCII, look the hex values up in the ASCII table. For your example:
is
A convenient way to get the table is
man asciifrom a terminal prompt.