Rich,
I'll second the recommendation for Dr. Pierre Picouet's "Tercio" site, it is one of my most frequently referenced sites. And it does include some information on the French, Dutch and English as well as the Spanish. There is a new link for it though, as he has had difficulty with the Geocities site hosting and has changed provider's. The new link is:
link
For the French, I would recommend two articles by Curt Johnson on the Xenophon Group's site (the Early Modern Warfare Society, former publisher's of "Gorget & Sash" magazine). They can be found here by scrolling to the bottom of the page for the two articles by Curt:
xenophongroup.com/EMW/EMW.htm
I would also recommend an excellent article that Bill Boyle did that is available for download on the Vexillia web site. Go to the link below and then click on the link on the left for "TYW Uniform Guide".
vexillia.ltd.uk/news.html
Next, I would highly recommend Stephané Thion's excellent book "French Armies of the Thirty Year's War" which is available through either On Military Matters or from Caliver in the UK. It contains a fantastic collection of period artwork depicting French troops into the 1650's.
As has already been stated though, "uniforms" as we think of them did not become standard until after the dates you specify. In the case of the French, 1668 is given as the official date for foreign regiments and 1670 for "native" French regiments, although individual colonel's would have uniformed their units to some degree if they chose to. There is a very prominent painting in Stephané's book of the French at Rocroi in 1643 in which one battalion of the Gardes Francaises are shown in all dark blue uniforms, although these were officially changed to gray in 1661. In Curt's article on the French line infantry, he indicates that there was one German regiment in French service, Clérac, that was uniformed in gray coats with reversed lining/facings as early as 1659 in imitation of the English troops that were fighting for Schomburg in Portugal. As to the Spanish, they were similar to the French in that the first official uniforms are not recorded until after 1660, and then only to the five senior, or "viejos" Tercio's, although there were some specific units uniformed by their colonel's as an individual matter.
Prior to the official uniforming of troops, the most common element on the Continent was the "field sign" agreed upon by the various powers. For the French this was a white sash worn by all troops, but in various and different ways (over either shoulder, around the waist, etc.). For the Dutch it was orange, and for the Spanish it was red. It was also not unknown for Spanish troops to be issued cassocks or cloaks with a red Cross of Burgundy and to wear red plumes.
One other prominent exception to the above that I am aware of, besides the English under Schomburg, is the English New Model contingent that fought under the French (Turenne) at the 2nd Battle of the Dunes (Dunkirk) in 1658, which were uniformed in red coats. However, there were also English Royalists fighting in the Spanish army at that battle that were not recorded as uniformed.
Bill
Sir William the Aged
warsoflouisxiv.blogspot.com