I’ve been looking at pictures of various heraldries, surcoats the like of knights and footmen around the 100 years war for inspirational purposes. I noticed that quite a few of these show gold on the fabric. Golden lions on red and gold fleur de lis on blue, would they have been golden in colour ? Or is this artist impressions and they would have in fact been yellows?
It's hard to say for any given image, but "cloth of gold" was certainly known and used during the time, if not necessarily accessible to the everyman. It typically refers to a very thin gold covering around a thread core (with varying specific methods of construction) which is then woven into fabric: hence, cloth of gold.
From the English side... Household accounts of Edward IV mention a brigandine (a type of armour made of many small plates inside a fabric cover) of cloth of gold being given as a gift, for example. Thom Richardson's PhD manuscript inventorying the armouries of the Tower of London notes a number of occurrences of cloth of gold in armour: in 1344 Robert Mildenhall received many pairs of cuisses and poleyns (thigh and knee armor) from John Fleet, one of which was covered in cloth of gold, another in red silk with the arms of England (though it isn't noted what specific colour or fabric the arms are): in 1353, William Rothwell also received a pair of cuisses and poleyns of cloth of gold, decorated with latten (specific decoration unspecified). Things such as red leather pairs of plates or red velvet horse horse bardings, both with the arms of England are also mentioned, though the construction of the arms isn't indicated. Note that these are all armour pieces, and none surcoats: that, specifically, I don't have any information on. Unfortunately, my high school French didn't leave me quite proficient enough to sift through French sources, but we at least know that cloth-of-gold armour existed in some capacity around the period on the English side, and the technology wasn't exclusive to England: William of Bavaria's 1357 inventory also lists a pair of plates covered in cloth-of-gold, for example.
Yellow/gold-dyed silk and velvets (or indeed other fabrics) could, of course, also be what you're seeing in many cases. Cloth of gold would have been outside the budget of many, with velvet or silk being much more affordable.
Metal threads: the historical development by Anna Karatzani
Wardrobe Accounts of Edward IV
The medieval inventories of the Tower armouries 1320-1420 by Roland Thomas Richardson