最近Emacs + ruby-mode を使用し始めたのですが、シンタックスハイライトに若干不満があります。

ruby-font-lock-keywordsで色付けの設定ができます。

僕の設定はこんなのです。参考になれば…

(setq ruby-keywords
      '("alias" "and" "begin" "break" "case" "catch" "class" "def" "do"
        "elsif" "else" "fail" "ensure" "for" "end" "if" "in" "module"
        "next" "not" "or" "raise" "redo" "rescue" "retry" "return" "then"
        "throw" "super" "unless" "undef" "until" "when" "while" "yield"
        ;; ^QRakefile keywords^P
        "task" "file" "desc" "rule"
        ;; ^Qelispunit / el4r keywords^P
        "def_test"
        ;; ^Qel4r keywords^P
        "defun"
        ;; ^Qasserts^P
        "assert_block" "assert_equal" "assert_in_delta" "assert_instance_of"
        "assert_kind_of" "assert_match" "assert_nil" "assert_no_match"
        "assert_not_equal" "assert_not_nil" "assert_not_same" "assert_nothing_raised"
        "assert_nothing_thrown" "assert_operator" "assert_raises" "assert_respond_to"
        "assert_same" "assert_send" "assert_throws" "assert"
        ;; ^Qrspec^P
        "context" "specify" "it"  "should" "should_not"
        ;; ^Qexpectations^P
        "expect"
        ;; other keywords 追加してください。
        "puts" "print"
        )
;;;;;
      ruby-highlight-keywords
      '("describe" "Expectations")
;;;;;
      ruby-font-lock-keywords
      (list
       ;; functions
       '("^\\s *def\\s +\\([^( \t\n]+\\)"
         1 font-lock-function-name-face)
       (cons (concat
              "\\(^\\|[^_:.@$]\\|\\.\\.\\)\\b\\(defined\\?\\|\\("
              (regexp-opt ruby-keywords)
              "\\)\\>\\)")
             2)
       `(,(concat
              "\\(^\\|[^_:.@$]\\|\\.\\.\\)\\b\\("
              (regexp-opt ruby-highlight-keywords)
              "\\>\\)")
             2 font-lock-warning-face prepend)
       ;; assignment
       ;; (regexp-opt '("=" "+=" "-=" "*=" "/=" "%=" "**=" "&=" "|=" "^=" "<<=" ">>=" "&&=" "||="))
       `("\\(?:&&\\|\\*\\*\\|<<\\|>>\\|||\\|[%&*+/|^-]\\)?=>?" 0 font-lock-warning-face)
       ;; xmpfilter
       '("# =>.*$" 0 font-lock-warning-face prepend)
       '(ruby-font-lock-xmpfilter-multi-line-annotation 0 font-lock-warning-face prepend)
       `(,(regexp-opt '(">=" "<=" "<=>" "==" "===" "!=" "=~")) 0 font-lock-builtin-face t)
       ;; variables
       '("\\(^\\|[^_:.@$]\\|\\.\\.\\)\\b\\(nil\\|self\\|true\\|false\\)\\>"
         2 font-lock-variable-name-face)
       ;; variables
       '("\\(\\$\\([^a-zA-Z0-9 \n]\\|[0-9]\\)\\)\\W"
         1 font-lock-variable-name-face)
       '("\\(\\$\\|@\\|@@\\)\\(\\w\\|_\\)+"
         0 font-lock-variable-name-face)
       ;; embedded document
       '(ruby-font-lock-docs
         0 font-lock-comment-face t)
       '(ruby-font-lock-maybe-docs
         0 font-lock-comment-face t)
       ;; ^Q"here" document^P
       '(ruby-font-lock-here-docs
         0 sh-heredoc-face t)
       '(ruby-font-lock-maybe-here-docs
         0 sh-heredoc-face t)
       `(,ruby-here-doc-beg-re
         0 sh-heredoc-face t)
       ;; general delimited string
       '("\\(^\\|[[ \t\n<+(,=]\\)\\(%[xrqQwW]?\\([^<[{(a-zA-Z0-9 \n]\\)[^\n\\\\]*\\(\\\\.[^\n\\\\]*\\)*\\(\\3\\)\\)"
         (2 font-lock-string-face t))
       ;; symbols
       '("\\(^\\|[^:]\\)\\(:\\([-+~]@?\\|[/%&|^`]\\|\\*\\*?\\|<\\(<\\|=>?\\)?\\|>[>=]?\\|===?\\|=~\\|\\[\\]=?\\|\\(\\w\\|_\\)+\\([!?=]\\|\\b_*\\)\\|#{[^}\n\\\\]*\\(\\\\.[^}\n\\\\]*\\)*}\\)\\)"
         2 font-lock-reference-face)
       ;; constants
       '("\\(^\\|[^_]\\)\\b\\([A-Z]+\\(\\w\\|_\\)*\\)"
         2 font-lock-type-face)
       ;; ^Qexpression expansion^P
       '("#\\({[^}\n\\\\]*\\(\\\\.[^}\n\\\\]*\\)*}\\|\\(\\$\\|@\\|@@\\)\\(\\w\\|_\\)+\\)"
         0 font-lock-reference-face t)
       ;; warn lower camel case
                                        ;'("\\<[a-z]+[a-z0-9]*[A-Z][A-Za-z0-9]*\\([!?]?\\|\\>\\)"
                                        ;  0 font-lock-warning-face)
       ;; ^Qeev hyperlink^P
       '("^ *#[^(\n]+\\((.*)\\)$" 1 ee-link-underline t)
     
       ))

ruby-mode.el（仮称）とかを編集すればできると思います。