Do you want all positive values above the threshold to get a constant color, and all negative values to get a different constant color? Or do you want the colors to vary once something is above threshold (which is the way the AFNI interactive slider would work)?

In the former case, you need to assign different values in the 3dcalc expression when 'a' is above 4.76 and when 'a' is below -4.76. This could be done with something like

3dcalc -a 'ANOVA_N24+tlrc[11]' -datum short -expr 'ispositive(a-4.76)-isnegative(a+4.76)' -prefix words_vs_fix_00001

a>4.76 is equivalent to a-4.76>0, thus 'ispositive(a-4.76)'

a<-4.76 is equivalent to a+4.76<0, thus 'isnegative(a+4.76)'

The resulting dataset will be +1 for a>4.76, -1 for a<-4.76, and 0 for -4.76<=a<=4.76. The '-datum short' option is mostly to ensure that the result is NOT a byte dataset, which can't store negative numbers.

The 'isnegative' and 'ispositive' functions were added to 3dcalc.c on 21 Oct 2004, so if your AFNI is so ancient, you should upgrade NOW NOW NOW.