This is a bug, or perhaps more accurately the 'lack of a feature' ...the meters are a special case when it comes to redraws, so things that work elsewhere not working with meters is too common for any example to strictly be describable as 'unexpected'.
Reaper minimises recalculating things, and its being efficient by not checking whether something has changed in the meters during normal use. Record arm always causes changes in the meters, so that is when Reaper examines the meters and sees that you have done a ?trackcolor_valid check, or that the custom color has changed, or whatever.
You could report this as bug, that would be valid. However, equally valid would be the response that you, as a themer, could avoid this by not trying to do that