[thEsp]

#include <amxmodx>

#define MAX_SAY_TEXT_LEN 128
#define MAX_KEYWORD_LEN 32
#define MAX_HIGHLIGHT_LEN 32

enum Match
{
    Keyword[MAX_KEYWORD_LEN],
    Highlight[MAX_HIGHLIGHT_LEN]
};

new g_ChatKeywords[][Match] =
{
    {"Relaxing", "^4Rela^3x^4ing"}
};

public plugin_init()
{
    register_clcmd("say", "@OnCmd_Say");
}

@OnCmd_Say(id)
{
    new szText[MAX_SAY_TEXT_LEN];
    read_args(szText, charsmax(szText));

    remove_quotes(szText);
    trim(szText);

    if (!szText[0])
        return PLUGIN_CONTINUE;

    for (new iPattern = 0, szHighlight[MAX_HIGHLIGHT_LEN], iPatterns = sizeof(g_ChatKeywords); iPattern < iPatterns; iPattern++)
    {
        if (containi(szText, g_ChatKeywords[iPattern][Keyword]) == -1)
            continue;

        format(szHighlight, charsmax(szHighlight), "%s^1", g_ChatKeywords[iPattern][Highlight]);
        replace_all(szText, charsmax(szText), g_ChatKeywords[iPattern][Keyword], szHighlight);
    }

    // %n was added on 1.9, but it directly parses a certain client's name.
    client_print_color(0, id, "^3%n%s^1: %s", id, is_user_alive(id) ? "" : " ^1*(DEAD)*^1", szText);
    return PLUGIN_HANDLED;
}

Take in mind this is just an example and there are other flaws.