Changing drupal_set_message red or yellow I am looking to change $errors[] = drupal_set_message(t(' Freshwater Shrimp should be a number!')); 

Into a red or yellow message at the top of the page. Looking at the doc I should do this in drupal 6

 $errors[] = drupal_set_message(t(' Gammarus (Freshwater Shrimp) should be a number!', 'Warning',));


But I have no luck.
You've put warning as a 2nd argument for function t(). Look carefully at the syntax. It should be an argument to the drupal_set_message function instead: 

Wrong:

 $errors[] = drupal_set_message(t(' Gammarus (Freshwater Shrimp) should be a number!', 'Warning',));


Correct:

$errors[] = drupal_set_message(t(' Gammarus (Freshwater Shrimp) should be a number!'), 'warning');