Update a few cases where array indices are assumed

[brenhinkeller]

This may not be everything, but a few simple cases that were easy to update

[codecov[bot]]

Codecov Report

Merging #521 (57a24b3) into master (b550b7b) will decrease coverage by 0.23%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #521      +/-   ##
==========================================
- Coverage   94.33%   94.09%   -0.24%     
==========================================
  Files           9        9              
  Lines        1270     1271       +1     
==========================================
- Hits         1198     1196       -2     
- Misses         72       75       +3     

Impacted Files	Coverage Δ
src/colormaps.jl	96.87% <100.00%> (+0.02%)	:arrow_up:
src/utilities.jl	95.23% <0.00%> (-1.10%)	:arrow_down:

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[Roger-luo]

@johnnychen94 should this be merged too?

[brenhinkeller]

Hooray!