How would a cavity in between both 2 front teeth be fixed if there is a very tiny space (~.5 mm) in between? Would it be filled to match the teeth?

Yes and yes. Cavities are like icebergs, most is under the surface. If you can see cavity it probably is fairly extensive. Yes, fixable. Yes, color match is part of the restorative procedure. No, you should not wait. The longer you put off treatment, the harder to fix. Please call Dentist right now. You'll be very glad you did.
Yes. There are a number of different shades of restorative material to make your restorations picture perfect.
Composite fillings. Dentists are pros at restoring carious teeth whether the filling is small or large. Composite fillings can be matched to the tooth so that it is imperceptible to the human eye at least by other individuals.
Dentist. Let you dentist figure that out. She/he has seen similar issued before and will be able to address that. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.
The decay. Would be removed and the cavity filled with matching tooth colored material. You would have to look hard to see it.
Very Carefully. The decay in the two front teeth would be carefully removed by your dentist and replaced with a composite resin material that would match your teeth.