A couple new environment variables have been added to control crosshair behavior in the AFNI GUI via, for example, your ~/.afnirc file. Well, technically, speaking these are old environment variables that were rediscovered---a fascinating note for future historians. But this also means that these are available in essentially any version of AFNI already.
These include being able to set the crosshair gap, as well as the crosshair color.
The crosshair gap is controlled now by AFNI_CROSSHAIRGAP:
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Variable: AFNI_CROSSHAIRGAP
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This numeric variable lets you set the gap between the image viewer
crosshairs. The default value is 5 voxels. Allowable range is from -1
(closed) to 19 voxels.
A value of -1 leaves no space between crosshairs; a value of 0 leaves the crossing voxel visible, and any value>0 increases the gap by the stated number of spaces in all directions.
So, a user could put the following into ~/.afnirc:
AFNI_CROSSHAIRGAP = 0
... which might actually become the default GUI setting in the near future.
The crosshair color is controlled in an integer-based specification style.
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Variable: AFNI_OVCROSSHAIR
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This variable controls the color of the crosshair in the image windows
in Ye Olde Style: users provide an integer that maps to a color. The
color-by-number mapping is as follows:
1: yellow 11: blue-cyan 21: rbgyr20_01 31: rbgyr20_11 41: salmon
2: yell-oran 12: cyan 22: rbgyr20_02 32: rbgyr20_12 42: navy
3: oran-yell 13: green 23: rbgyr20_03 33: rbgyr20_13
4: orange 14: limegreen 24: rbgyr20_04 34: rbgyr20_14
5: oran-red 15: violet 25: rbgyr20_05 35: rbgyr20_15
6: red 16: hotpink 26: rbgyr20_06 36: rbgyr20_16
7: dk-blue 17: white 27: rbgyr20_07 37: rbgyr20_17
8: blue 18: gry-dd 28: rbgyr20_08 38: rbgyr20_18
9: lt-blue1 19: gry-bb 29: rbgyr20_09 39: rbgyr20_19
10: lt-blue2 20: black 30: rbgyr20_10 40: rbgyr20_20
So, a user could put the following into ~/.afnirc:
AFNI_OVCROSSHAIR = 16
... and have hotpink color crosshairs by default.
NB: it is possible that there might be a new environment variable to control this in a more direct-color-naming kinda way.
Thanks go to @rickr for excavating these! 
