I have to display numeric values with a red down arrow or a green up arrow  depending whether the number has increased or decreased since last week numbers, a bit like a stock web part   

My initial idea was a list with 2 columns like this week number and last week number that the webpart might read from  

How do I make this in the easiest way?  - maybe it's possible by using a InfoPath form ? 

A bit like this one: http://spstockticker.codeplex.com/ but instead of green and red text it should use an image

make a form with two fields, and two sections

field one = Value

field two = previous value

add a formatting rule to this section to hide it if required, something like, if field 2 is blank OR if field 2 is not blank, hide

section one = contains an image Green Up arrow

add a formatting rule to section one, if field one IS NOT greater than field two then hide this section

section two = contains an image of a red down arrow

add a formatting rule to section two, if field one IS greater than field two then hide this section

Now when the form loads only field one will be visible with either a green or red arrow also displayed dependent on the previous value.

Or you could end up by simply using a calculate column in which you can put formula to apply color coding and the icons according to your conditions.

You can refer the below links to see how to implement color coding using calculated columns.

http://blog.pathtosharepoint.com/2010/01/04/html-calculated-column-and-unicode-graphics/