Here are a selection of tools that get used in most of my classes.
This seems to be the color wheel that most of my beginner students like to use. As one starts mixing colors, it is a great help to be reminded about "complimentary" colors. This one is pocket sized but the same wheel is available larger. Here is a link
The little window can be adjusted to be the relative dimension of the canvas (square, 8 x 12, 9 x 12, 12 x 16) marked on the frame. This can help in selecting the canvas shape. Then, by looking at a scene or a setup through the appropriately sized window, one can use this to frame and sketch or block in onto the canvas.
Here is a link
This tool also aids to choosing and sketching or blocking in a scene or set up. It also has a red filter to help see things in "grey scale". The one I have is folded and includes a printed grey scale.
I found the one shown here available on Amazon link
tI have some plastic pieces that open similar to this commercial true angle tool. This is very useful for checking angles (rooflines, nose to eyes) in drawing. One could make one with some strips of mat board and an old-fashioned acco fastener. Here is a link to purchase this one.
This little painting had the background modified to allow the dog to "take center stage". I am posting it to Daily Paintworks today.