Yes to picking a base colour for trips. She can have 2 sets (or more if she really wants) of base colours in her wardrobe at HOME, but for each trip base it around 1.
So black trousers and shoes, with different accent colours in tops or scarves or jewellery, and any dresses should have a black base rather than navy or brown.
So for 4 days, maybe black trousers with a black jacket and coloured top day 1, black and red dress day 2, trousers and a black top/jumper with no jacket but a coloured scarf or statement necklace day 3 and back to the jacket and trousers and a different top day 4 or a second dress.
Packed case includes 2 tops, a dress, pair of heels for the dress, underwear/tights and 1 or 2 scarves/pieces of big jewellery. Wash kit. PJs. And something for evenings - possibly gym kit if there are facilities, and also maybe a lounging trousers to wear with that day's top, or jeans if wearable with runners. That can fit into even a small carryon if you are careful in folding them - and packing cubes can help to split dirty from clean and also to have each days outfits ready to go rather than having to unpack everything at once.
And for another week, if she has the wardrobe to do that, she could do a similar type of packing but based around navy or grey or brown and with navy/black?/brown shoes and handbag. But possibly able to use many of the same accent tops and scarves and jewellery.
I keep my travel wash kit packed, and repack it as soon as I get home rather than when I am setting off again.
Places like Muji sell travel bottles in a range of sizes - so buy some to decant shampoo, conditioner, shower gel, cleanser, etc into. I mark mine with a letter, in different coloured nail varnishes, to tell what's in each. Use travel sized toothpaste - muji sell a tube to fill if her favourite is not generally available in travel size.
I use 30ml sizes for shampoo/conditioner for overnight/2 night trips, but 100ml for 2 weeks. And usually hotel shower gel at least is ok if you do forget yours.
Perfume decant into a small dispenser, and when she's buying her regular moisturisers/cleansers etc ask for sample sizes as well as they are great for travel use - especially if she uses any "big brand" names (I use Estee Lauder and mostly go to the same Dept store for mine). Some samples are single use sachets, while some are small bottles or tubs and last a while (and those can usually be refilled from the big sized containers at home as well).
I also keep a decent scrubby sponge (little round exfoliating type), some cotton wool pads, a few cotton buds, a tweezers, nail scissors (if not flying), small hair brush and nail file in mine. And while deodorant is not possible to decant, I use a full one to about halfway gone at home, and then transfer that to the travel kit to cut down on weight.
And I have a full second set of makeup which lives in my washkit ready to go.
Also look for some bits of luxury in single use - hair conditioning treatment, face mask, black head remover strips, serum masks, hand scrub and hand cream...to be able to treat yourself on some nights when sitting with not a lot to do.
If there is the potential for her trips to extend while she's already on the road, I would add a spare bra, 1 spare knickers and tights/socks, and maybe 1 spare top if she has space, or just a spare scarf/jewellery item, and bring some travel wash (and maybe a travel washing line - it takes up no space, pouch for earbud type headphones, but can be handy to get things dry in limited space bathrooms!). And in winter, add thermal vests if necessary - but wash day 1 to be able to reuse on day 3/4.
I use this case as DH got it a few years back, and it holds plenty for up to 3 days (and I tend to pack heavy!), DH takes it for 6 days. Before that I used a Samonsite an older version of this basic model which actually has more room but everything is in 1 compartment and it is more cumbersome so I only use that on longer trips now.
And as someone mentioned kettle - I usually bring a handful of my favourite tea bags and a couple of 2 packs of biscuits for late evening (rather than always going to the bar for wine), and buy a pint of milk. It's rare for me to find them, but I stock up on the boxes of 2 packs of chocolate digestives and chocolate hobnobs when I do see them, or maybe get a single piece of chocolate biscuit cake from another maker that does individual slices pre-packaged. And I keep a couple of single packs of dried fruit and nuts from Lidl and bar of plain dairy milk for nibbling as well during the day or night. I do TRY to get fresh fruit when travelling as well, to offset the pastries for breakfast and with coffee and not eating the same way I can at home.