Yes, You Can Use A Red Onion When You Don't Have White Ones!
Go ahead: chop and use it.
It's hard to substitute ingredients when you're a beginner cook. Sometimes, it's the reason for a dish not working out because the original ingredient may be different from what it should have been. Then, there are recipes which aren't as specific as you would like and leaves you wondering if the onion you're using is okay to use.
In the Philippines, the most common onion available in markets is the red onion followed by the yellow (mistakenly thought as white) onion and the shallot. Since the red onion is the most available, it's the natural substitute for recipes which call for an onion. But while substituting ingredients isn't always best, it's okay to use a red onion in a recipe that says to use the yellow or white onion or doesn't state which type of onion to use.
Here are two reasons why the red onion is fine to use in your recipe:
1 Red onions are sweet, too.
While both can and may make you cry when chopped, between these onions, the red is the mildest of the two in flavor. Both are sweet and while the preferred onion for caramelizing is the white or yellow because it is easier to see the golden-brown color, the red onion also becomes sweet once cooked.
2 Red onions are best if using raw.
Not only are red onions a gorgeous violet hue, it's the preferred onion in salads, in pickles, and in any dish which uses a raw onion. That's because red onions are also milder in flavor when raw than the sharp yellow onions and won't leave you with onion breath.
The best reason to use a red onion for a recipe is probably that it's what you have on hand. For recipes which call for the yellow onion and what you have is the red one, what you should consider is if it will change the way the dish will look because red onions really are more visibly arresting than the white. If the dish will be served raw or not, take into account whether you prefer raw red onions or white onions.
Keep these in mind and the next time you're cooking with onions, you're fine with whatever variety you have.