[Help-bash] Using slices with arrays

[Nick Chambers]

Hello all! I am using bash4.4 and have some questions about slices in bash. For 
my first example:

NickChambers-iMac:~ Nick$ str='this is a message'
NickChambers-iMac:~ Nick$ echo "${str:4:(-1)}"
 is a messag
NickChambers-iMac:~ Nick$

This clearly allows a negative index, but if I try to do the same thing with an 
array, I get the following:

NickChambers-iMac:~ Nick$ colors=(purple blue orange yellow green)
NickChambers-iMac:~ Nick$ echo "address@hidden:1:3}" # expected
blue orange yellow
NickChambers-iMac:~ Nick$ echo "address@hidden:1:(-1)}"
-bash: (-1): substring expression < 0
NickChambers-iMac:~ Nick$

Is there a reason why this can be done on strings but not on  arrays?
Also, incidentally, is there a better way to span from an index to the end of 
the string besides using something like the following:

NickChambers-iMac:~ Nick$ str='this is a message'
NickChambers-iMac:~ Nick$ strlen=${#str}
NickChambers-iMac:~ Nick$ echo "${str:4:strlen}"
 is a message
NickChambers-iMac:~ Nick$