Tip: my paintings include a red under layer which is visible in the final piece and a red frame compliments this beautifully.
8×10 inch:
https://www.ikea.com/ca/en/p/roedalm-frame-red-20566361/#content
12×16:
https://www.ikea.com/ca/en/p/roedalm-frame-red-20566361/#content
16×20
https://www.ikea.com/ca/en/p/roedalm-frame-oak-effect-80566400/
18×24
https://www.ikea.com/ca/en/p/roedalm-frame-for-9-pictures-black-70553709/