At minimum, you will need the following eight tubes:

Purple-biased Blue: Look for PB29 on the label French Ultramarine 263, Winsor & Newton Artists; Ultramarine Blue 219, Grumbacher Academy. NOTE: buy a large tube of this color or double up.

Green-biased Blue: Look for PB15 or PB15:3 on the label Winsor Blue (green shade), Winsor & Newton Artists; Thalo Blue, Grumbacher Academy

Green-biased Yellow: Look for PY3 on the label Winsor Lemon, Winsor & Newton Artists; Lemon Yellow, Grumbacher Academy

Orange-biased Yellow: Look for PY97, PY150 or PY154 on the label Transparent Yellow, Winsor & Newton Artists; Golden Yellow, Grumbacher Academy

Purple-biased Red: Look for PR206, PR 176 or PV19 on the label Permanent Alizarin Crimson, Winsor & Newton Artists; Thalo Crimson, Grumbacher Academy

Orange-biased Red: Look for PR188, PR253 or PR108 on the label Scarlet Lake, Winsor & Newton Artists; Vermilion Hue, Grumbacher Adademy

Earth Tones: Burnt Sienna: Look for PBr7 on the label; Yellow Ochre: Look for PY42 on the label.

Additional Colors could Include: Ivory Black: Look for PBk9 on the label Cobalt Blue: Look for PB28 on the label. Cerulean Blue: Look for PB35 on the label. Chrom. Oxide Green: Look for PG17 on the label. Permanent Magenta: Look for PV19 on the label. Raw Umber: Look for PBr7 on the label.

NOTE: Tube watercolor sets available online at outlets such as Dick Blick can be substituted - keep in mind the color/brand recommendations above. Pan sets can be used to expand the above palette. But these are not a good choice when developing an area of flat, even washes.