you have conflicting rules
this rule:
RewriteRule ink$ products_results.php?Search=1&ProductCategoryID[]=2
is overwriting this one:
RewriteRule fluorescent_ink$ products_detail.php?ProductID=118
because they both end with ink
mod rewrite is not my specialty, so i am not entirely sure of the exact syntax, but I believe it should use:
RewriteRule ^ink$ products_results.php?Search=1&ProductCategoryID[]=2
RewriteRule ^fluorescent_ink$ products_detail.php?ProductID=118
to anchor the expression to the start of the string.