How did the German Wermarcht employ Forward Artillery Observers during the Blitzkrieg? (Notably during the Invasion of France) What level were they attached out at, Company, Brigade, Higher?

Each infantry division had a single artillery regiment composed of 3-4 battalions each of which would usually fire as a unit. The allocation of artillery support to the division's infantry regiments would vary but typically each regiment of three battalions, each of around 800 men, would enjoy the support of a single 16-gun artillery battalion.

The limiting factor was usually the availability of shells and firing a few rounds in a short space of time is more effective than dragging the same fire out so fire for effect would normally be done by all guns at once. On other occasions, shells would be dripped 1 at a time over a prolonged period but it would be rare for batteries from one battalion to fire separately.

An observer needn't always have been present for the battalion to fire. If a given spot such as an obvious enemy approach route had been zeroed already -- i.e. "pre-registered" -- then fire could be requested by the infantry even if no observer was present. More rarely, fire might be ordered without observation or correction but this was regarded as very wasteful except for map fire on targets in non-observable positions which was in any case uncommon except for harassment of enemy batteries or during high-intensity battles like major offensives.

Additionally, would these men be members of the Infantry or Artillery arm?

The artillery.

What color waffenfarbe would they be wearing if they were attached out to the infantry.

No idea, sorry, but these were specialised artillery personnel and not infantry if that helps.

Finally, would they be responsible for their own communications with their battery or would a Signaller be attached with them as well.

Normally in the infantry divisions they were reliant on telephone communications AFAIK. If they had a radio then at least one assistant would be needed as the batteries and sets were bulky, but in this case it would be another person from the artillery regiment. Specialist signals units provided HQ communications and AFAIK supported the other signallers in the regiment with spares, repairs etc.