Picture frame tips
You could also use a custom made passepartout to fit your crooked sized picture in a standard size frame. The passepartout is relatively cheap to make in custom sizes, so together with the cheap standard sided frame you will get a complete solution at a low price.
And standard size frames are in stock in a wide range, ready to order online with or without passepartout. Especially a square frame is well suited for this purpose.
And what color/type of frame should I choose?
To begin with it is always a good idea to assume that the color of the frame should be similar in style, but in contrast to the wall. So dark or black frames go well with white walls. Dark wooden frames go well with light wooden walls. White frames provide a good contrast with colored walls.
And choose a style that fits. That is, if you have rustic or antique furniture, choose a frame in the same style. If the interior is minimalistic, a thin, narrow aluminium frame is a good choice. If the furniture is robust and solid, then choose a style with old, wide, wooden frames. Is it modern, then go with narrow aluminium frames for framing.