think about winter flowering fragrant shrubs like Wintersweet, Sarcoccoca, some Osmanthes I think. Gorgeous wafting scents that fill your dooryard. Blooms in January and February. Some say Wintersweet begins in Dec.

Witch hazel winter blooming wafting fragrance, big bush or small tree.

Also early blooming bulbs like snowdrops and aconite will bloom in February. Hellebores are winter bloomers, but many have green flowers.

My hardy cyclamens happily bloomed along the path all winter, happy little pink and white things.

It's hard to go wrong with Sasanqua Camellias like 'Yuletide', which I particularly like because it is easy to prune.

Where in north California are you?

M. D. Vaden of Oregon

Applegate Valley / South Oregon - soon to be in North Oregon

Part of the problem is limiting your impression of 'year round color and interest' to just flowers. While our mild climate offers a good number of plants that will bloom in the off-seasons (late fall to early spring), there are far more that will provide an equal amount of interest through variations in foliage color and texture or through colored or interesting stems and bark or berries.

For northern California, I wouldn't discount things like strawberry tree (Arbutus unedo), silk tassel (Garrya elliptica), nandina, phormiums, libertia, euphorbias, manzanita, edgeworthia and a whole host of evergreen shrubs, grasses and perennials. For fall color, visit the nurseries in September and October. This is a prime planting time for most of the west coast (even better than spring) and you are far more likely to see both plants that flower at that time of year as well as those that offer fall foliage color than you are shopping only early in the season.

The only thing good about planting in spring is that outlets have their best selection. Otherwise is is a lousy time to plant hardy plants (tender plants would of course die if planted at the otherwise preferred time in fall).

In spring soils are cold and soggy, top growth is underway or soon will be and summer drought is right around the corner.

Sunset Western Garden Book shows flowering times of numerous kinds of plants in the plant selection guide near the front. Otherwise I find the lists near the back of The Hillier Manual of Trees & Shrubs to be pretty good. Here's a sample (from the part called 'Flowering Trees and Shrubs for Every Month'):

July

Trees

Aesculus indica
Castanea sativa
Catalpa
Cladrastis sinensis
Eucryphia
Koelreuteria paniculata
Liriodendron tulipifera
Magnolia delavayi
Magnolia grandiflora
Stewartia

Shrubs

Aster albescens
Buddleja davidii*
Calluna vulgaris
Cistus
Colutea
Daboecia cantabrica
Desfontainea spinosa
Deutzia setchuenensis
Erica ciliaris
Erica cinerea
Erica tetralix
Erica vagans
Escallonia
Fuchsia
Grevillea juniperina 'Sulphurea'
Halimodendron halodendron
Hebe
Hoheria
Holodiscus discolor
Hydrangea
Hypericum
Indigofera
Lavandula
Magnolia virginiana
Olearia
Penstemon
Philadelphus
Phygelius
Potentilla
Rhododendron
Romneya
Yucca
Zenobia

Climbers

Clematis
Eccremocarpus scaber
Fallopia baldschuanica
Jasminum
Lonicera
Mutisia oligodon
Passiflora
Schizophragma
Solanum
Trachelospermum

*Now considered a weed in this region

I like to plant so that there is a succession of bloom from early spring until fall-
early Spring
Hellebores, species Tulips, Daffodils, Pulmonarias
Spring
Aquilegia, Ajugas, Saxifrage, Peonies, Bleeding Hearts, Irises (particularly Tall Bearded), Roses, Clematis, Hardy Geraniums, Rosemary, Epimedium, many shrubs
Summer
Roses, daylilies, Phygelius, Fuchsias, Hardy Geraniums, Lilies, Dahlias, Clematis, Japanese Anemones, many perennials, annuals
Fruit, veg- (you have to eat too) Apples, Pears, Blueberries- evergreen varieties and regular have beautiful fall color, Raspberries tomatoes, squash, beans,
Fall
Roses, Fuchsias, grasses, Dahlia, Nandina for fall color

This is a zone 7-8 list so you could grow way more things

One neglected winter color is what I call "better green".

At our next home (just a week or so away), I plan to replace the less than prime front lawn, and improve the soil, with a new sod lawn cover.

A well tended and properly "fed" lawn, can provide a much more pleasing green color in winter. Not perfect, but rather nice to compensate for deciduous foliage falling.

Doubt I'd plant it, but there is some Cryptomeria that turns reddish of purplish in winter.

I plan to plant a decent group of blue fescue, one of my favorite plants to look at in winter.

M. D. Vaden of Oregon

The cryptomeria is 'Elegans'.