Colour by number is a well known childhood activity and in most cases requires no maths other than number recognition.
Take for example this Halloween Scarecrow picture, which I have completed online using the Color It by Numbers website here. As you can see each number represents a particular colour and once finished the image is more defined.
To add difficulty to this activity each number can be replaced by a question with that numerical answer. For example this pumpkin colour by number requires students to solve the equations to find the value of x. Each answer is then one of the five colours. This worksheet can be downloaded from Education.com, which is a free account based website.
I am still not satisfied that there is enough challenge for all pupils, in which case I use a blank template such as this bat picture below taken from Coloritbynumber.com here, and ask pupils to create their own question with the correct numerical values for which ever topic is most relevant. 
You could ask pupils to create a set of questions limited by topic area for example; BIDMAS, solving equations, area, perimeter, evaluating formulae, alternatively pupils could create it based on a number of topics recently studied. Their work (when checked) could then be given an a starter of homework activity for another class.
Other Halloween Activities we Love
Relay Races
We love relay races as a great team and review activity, check out our blog post here about how to run a relay race. A collection of relay races for all occasions, not just Halloween, can be downloaded from here made by Chris Smith @AAP03102.
Skeleton Rotational Symmetry
Check out our post on making some Halloween decorations using rotational symmetry here.
Witches Brew
Check out our blog here.
Thank you for reading NumberLoving!
Sharon and Laura
