The red pigment - iron oxide - is an abrasive used in glass shops to polish windshields. I don't reccommend it unless you're willing to frequently re-scape and calibrate your reference tooling. Rub out a film on the work and you'll see the red color soon gets muddy with abraded cast iron. Red iron oxide is a very effective lapping compound.
You can get red lead pigment from suppliers in sample amounts make a cake large enough to scrape in a Panama Canal lock gate. If you do use red oxide, spread it with an ink roller (caled a "breyer" in the printing trade for some reason).
A word about rollers Vs daubers. Daubers (rolled beltng, leather pads, fabric knots, stencil brushes, etc) are time wasters. Draw from your own experience and compare with this: I can ink a 3 ft x 4 ft surface plate from scrach in about 40 seconds (my students have seen me do it) and it leaves no crumbs, fuzz, or threads to screw up the print. And at the end of the day my wrists and finger joints don't ache with the swirling motion needed to produce a thin uniform film.
Breyers (ink rollers) are available from most any art supply store along with a wide selection of dry powder paint pigment and intaglio inks specially formulated to stay put in a humungous array of colors.
If you insist on old school. here's a seller of dry red lead pigment. They might offer samples.
Lead Powder,White Lead Powder Manufacturer,Red Lead Powder Suppliers from India
One reason why red lead was so popular is the stuiff is more of a solid lunricant. It also has a vivid color and spreads to very thin films. The grains are very soft and smoosh without abrading. It is toxic but it's also the best ever for a scraping contrast medium. Don't lick your fingers. Keep it off your sandwiches.
I always bought my pigments and breyers from Mister Art but a large local art supply will have what you need.
Here's a website for the rollers:
Speedball Rubber Brayer | MisterArt.com
I don't know what you have for black. If it's carbon or graphie, fine, but it's a bugger to mix. Chances are it's manganese dioxide and other stuff. Manganese dioxide is also abrasive but not as aggressive as iorn oxide.
Have you looked at the Canode line sold by Dapra Corp:
Power Scraping Accessories & Aids | DAPRA
Here's something I wrote a few years ago pertaining to spreading blue:
One can with care and a little math predetermine film thickness of scraping media by deliberate control of the blue to be spread:
Starting from a clean surface plate, use a hard rubber printer's roller (breyer) to spread the blue. Calculate the area of the surface plate and multiply by 20 millionths. For example: 18" x 24" = 432 sq inches x 0.000020" = 0.0086 cu inch. A nerdle (official term for a blob of any kind of paste or gel issued from a tube) of Prussian blue if stretched a bit as its squeezed from the tube is 1/8' dia. If your eye needs a little help, use a short length of 1/8 wire for visual comparison. 1/8" dia = 0.016 sq inch. 0.0086 cu inch / 0.016 sq inch = 0.54"
Therefore a 1/8" dia nerdle of Prussian blue a bit more then 1/2" long uniformly spread over an 18" x 24" plate will results in a 0.000020" thick film of blue (starting from a clean plate) and counting the area of the roller. When spreading the blue, roll briskly, lifting the breyer at the end of each stroke so it spins to randomize the blue spots it lays down.
This film thickness imparts a strong but transparent blue color. If you keep this color in mind, you can refresh the blue several times before the thickness starts to drift. BTW, 20 millionths thickness is best suited for finishing; it's too thin for initial scraping prints. Use 40 - 60 millionths for general rough scraping. .
Metricoids: there's about 40 millionths of an inch to a micron or 0.001 mm.
Do not use a dauber. It takes up varying amounts of blue making rough quantifying the film thickness almost impossible. Use a rubber printer's roller (NOT a foam one) as it is non-absorbent and its small area has little effect on the film thickness on the much larger plate area.
Do not use your fingers to spread blue, the vigorous rubbing necessary puts heat in the plate and the circular motion is hell on your finger joints.
I noticed discussion of alcohol as a "hazing" agent used with a clean (unblued) flatness reference. I don't recommend alcohol. or any cleaner with a high heat of evaporation. When it evaporates it carrys away significant heat contracting the face it wetted. While the haze it leaves behind is easily visualized and very thin, it comes at the price of having to wait a few hours for thermal equilibrium to wend its slow way through-out the mass of the scraped workpiece before a print can be taken. Instead I recommend a fast rub with the heel of your hand to leave a thin coat of skin grease. If done with a rapid wiping motion problems from heat input are reduced to nearly zero, and the haze seen in grazing light is as visible as the alcohol haze. Us older farts may have dry hands deficient in skin grease. If that's the case, use the inside of your forearm being careful of shedding arm hair.
When you look at the printed surface in grazing light you see the haze dulling the flash and sparkle of the scraped surface. Scattered in the haze are little bright pinpoints indicating the bearing points. Tracking down and scraping these points is a truly heroic task requiring immense patience and persistence; the process is called "pinpointing" (surprise). The result of pinpointing when carried to its logical conclusion with excruciating temperature control is a scraped surface in conformance to its master reference surface within small millionths.
I encourage any advanced scraping beginner to go through this pin-pointing ordeal at least once as a rite of passage. Having done it once, there is seldom a need to do it again unless you are re-scraping precision gaging equipment like the bed of a Pratt and Whitney Super Mike. Scraping references and high end machine tools are not improved in perceivable accuracy or longevity by such refinements and they represent a significant waste of time when employed on general purpose machine tools